This is not good on any level. Time to add back in some carbs/calories!

I think this sums it up pretty well:

http://en.wikipedia.org/wiki/Amenorrhoea

Hypothalamic Amenorrhoea

*Women who go through significant weight loss, dieting, or perform considerable amounts of exercise on a regular basis are at risk of developing hypothalamic (or 'athletic') amenorrhoea. It was thought for many years that low body fat levels and exercise related chemicals (such as beta endorphins and catecholamines) disrupt the interplay of the sex hormones oestrogen and progesterone. However, recent studies have shown that there are no differences in the body composition, or hormonal levels in amenorrhoeic athletes as compared to regularly cycling athletes. Instead, amenorrhoea has been shown to be directly attributable to a low energy availability. Many women who diet or who exercise at a high level do not take in enough calories to expend on their exercise as well as to maintain their normal menstrual cycles.[6]

A second serious risk factor of amenorrhoea is severe bone loss sometimes resulting in osteoporosis and osteopenia. It is the third component of an increasingly common disease known as female athlete triad syndrome. The other two components of this syndrome are osteoporosis and disordered eating. Awareness and intervention can usually prevent this occurrence in most female athletes. Many doctors simply prescribe the combined oral contraceptive pill to women with hypothalamic amenorrhoea, and these women continue in their patterns of under-eating and over-exercising. Often, they do not realize that they have a problem until they are trying to conceive, when their amenorrhoea leads them to seek the treatment of a reproductive endocrinologist.[7]*

Sounds like your leptin secretion from your fat cells is too low, so your hypothalamus shut down GnRH production, which shut down FSH, LH and eventually estradiol. You just need to gain some body fat one way or another. Eat more fatty meats, avocado, macadamia nuts etc. and eat more carbohydrates since there's no reason for you to restrict them.

Edit: Yeah what Dragonfly said.

AFAIK not menstruating at the age of 21 is a serious problem, food-related or not. Maybe you should consult a doctor?

Yes, you are hurting your health. Amenorrhoea can cause significant bone loss and fertility problems later. It's definitely a sign that you need to eat more carbs (probably) and put some weight back on. Unless the idea of having osteopenia in your twenties/early thirties sounds fun.

JUST WANTED TO GIVE MY 2CENTS ON WHAT I HAVE READ...

The most common cause of the hormone deficiency leading to missed menstrual periods in young women is inadequate nutrition. Anorexia almost always causes amenorrhea. Many women who are not anorexic but who miss their periods have lower dietary intakes of fat, especially saturated fat, as well as protein and total calories, compared with women who menstruate regularly. Women with amenorrhea also tend to obtain a greater proportion of their calories from high-fiber carbohydrates. In preliminary studies of normal-weight women with no obvious eating disorders, those who reported missed periods had diets described as "close to normal" but significantly low in fat. These women also tended to have low percentages of body fat.

Amenorrhea also can result from excessive exercise. Women athletes and performers tend to diet to avoid weight gain. Combined with the increased nutritional demands of intensive exercise, dieting can lead to nutrient deficiencies and lowered body-fat percentages that lead to amenorrhea and bone loss.

Amenorrhea also has been linked to smoking and stress, but smoking cessation and relaxation do not necessarily cure it. The primary reason for treating amenorrhea is that it can cause bone loss that cannot be remedied by taking calcium and vitamin D. Amenorrhea also can result from potentially life-threatening disorders of the hypothalamus, ovaries, or pituitary gland.

Amenorrhea, though not a life threatening disease in and of itself, but it certainly needs attention. More importantly it causes mental trauma and physical pain to one who experiences it. This condition can be caused due to several reasons. Addressing the reasons causing Amenorrhea in a patient is the proper way to start the treatment

Hypothyroidism is a condition that describes an underactive thyroid gland and goes hand in hand with amenorrhea. This condition develops when the thyroid gland is not producing enough of certain important hormones. Hypothyroidism disrupts the optimal balance of chemical reactions within the body. It is difficult to detect in the early stages, but as time passes, the untreated condition can lead to several health problems including joint pain, excessive weight gain, osteoporosis, infertility and even heart disease. In some cases, hypothyroidism has been the underlying cause for amenorrhea. An imaging test may be administered by your doctor after reviewing your symptoms and blood test results. Imaging tests such as a CT scan (computerized tomography), an MRI (magnetic resonance imaging) or an ultrasound can show if there are pituitary tumors or any abnormalities in reproductive organs.

Birth control pills suppress the pituitary gland's production of the hormones FSH (follicle stimulating hormone) and LH (luteinizing hormone). These hormones help stimulate the follicles in the ovaries to develop. As the follicles grow, they produce estrogen. After they release an egg, they produce progesterone. Birth control pills contain estrogen and progesterone. If a woman is taking birth control pills, her body will believe that she is ovulating because of the estrogen and progesterone in the medication. Her pituitary gland will stop producing FSH and LH. Without these two hormones, ovulation does not occur and it is close to impossible to become pregnant (there is always a very small chance that birth control pills may not work for certain women). After taking birth control pills for a length of time, the pituitary gland may continue to believe that there is a sufficient amount of estrogen and progesterone even after the medication is stopped. This causes her body not to ovulate and subsequently not return to a normal menstrual cycle. This absence of a normal cycle can be the complete absence of a period or irregular periods.

Amenorrhea affects around 1% of women in their reproductive age and while the condition is a major cause for infertility, prolonged amenorrhea could trigger the irregular thickening of the uterus lining that sets the stage for endometrial cancer. Failure to menstruate may cause the uterine lining to thicken putting you at risk for cancers Typically, leaner women have higher levels of protein which affect estrogen; therefore, less free estrogen is available to stimulate the reproductive cycle properly. Other specific causes include stress, nutritional inadequacies, hormonal imbalances, tumors, intrauterine infection, endometriosis, discontinuation of oral contraceptives, diseases such as diabetes mellitus or tuberculosis, anorexia nervosa, or obesity. In hypergonadotropin amenorrhea, high gonadotropin â"EUR any of several hormones that stimulate the growth and activity of the gonads, especially follicle-stimulating hormone (FSH) and luteinizing hormone (LH) â"EUR and low estrogen levels create symptoms similar to those of menopause: atrophy of the reproductive organs, hot flashes, irritability and other changes. The patient may experience internal sensations of heat, thirst, irritability, low back pain, vaginal dryness or dryness of the eyes. This type is affiliated with polycystic ovary syndrome (PCOS), or premature ovarian failure (POF).

Women who have amenorrhea should limit their consumption of caffeinated coffee to 1-2 cups per day. Caffeine can contribute to osteoporosis. Vaginal dryness is an indication of low estrogen production, but the frequently recommended soy isoflavones, red clover, and black cohosh are not appropriate. Flax seed (not flaxseed oil) may be more helpful, when taken for 3 to 4 months. Amenorrhea with unusual hair growth is a symptom of polycystic ovarian disease (POD), and unusual hair growth in women athletes is a strong indicator of the condition. Treatments for POD overlap those for type 2 diabetes (see Polycystic Ovarian Disease). Osteoarthritis (OA) is also known as degenerative arthritis or degenerative joint disease. Degenerative arthritis consists of the mechanical abnormalities of a particular joint that leads to the decay and break down of the joint cartilage and bone. Degenerative joint disease can be primarily caused by hereditary, developmental inefficiencies, metabolic alterations, and mechanical stresses, and nutritional deficiencies. In many situations, osteoarthritis accompanies other conditions such as diabetes, inflammatory diseases (Rheumatoid Arthritis, Lyme’s Disease, Gout, etc), obesity, traumatic injury, and blood disorders.
Nutritionally, there are multiple minerals, vitamins and herbs than can give relief for a person that experiences pain and discomfort due to osteoarthritis. Although peer-reviewed literature occasionally demonstrates conflicting results when comparing nutritional and herbal therapies, clinical results prove the overwhelming effectiveness of supporting the natural biological process that is dependent on micronutrients such as vitamins and minerals. • Glucosamine sulfate: Glucosamine sulfate is used to stimulate the rebuilding of damaged cartilage. It is not known to interfere with anti-inflammatory drugs, although some companies process their glucosamine sulfate with sodium chloride which can negatively affect a person’s symptoms. • Vitamin C – Iron – Alpha-ketoglutaric Acid: These substances are required for hydroxylation of L-proline to L-hydroxyproline which is needed for quality collagen production. Some of the best foods to get these nutrients include chard, spinach, kidney beans, kale, papaya, bell peppers, and broccoli. • Manganese: Manganese is a trace mineral that’s important for tissue repair including skin, bone and cartilage. Manganese is also important for the natural production of the superoxide dismutase (SOD), a powerful antioxidant enzyme. According to Katherine Toye,”Manganese-based SOD helps prevent the deleterious effects of the super oxide free radical from destroying cellular components, and without it, cells are susceptible to damage and inflammation. Immediately after trauma, the body starts coding for more SOD1 to be produced, and repletion of manganese after inflammatory trauma is necessary to keep the free radical defense system in proper working order.” • Vitamin D and Calcium: Vitamin D is required for calcium absorption in the gut and maintaining calcium and phosphate concentrations to enable normal mineralization of bone and prevent hypocalcemic tetany (spasms that result from lack of minerals in the blood stream). Vitamin D is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts. Without enough vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis. There are a variety of nutritional factors involved with the prevention and treatment of osteoarthritis. A lifestyle and diet that negatively affects the nutritional requirements that bones and joints need will result in accelerated degeneration of the affected tissues. Nutritional requirements work best when found in a nutritional complex and a patient would benefit greatly by incorporating nutritional supplements in a whole food base and not as a single, isolated part.

Celery seed Celery seed contains high amounts of butylidene-phthalide which helps to balance woman's natural hormones and reduce emotional stress causing irregular hormone production by cleansing toxins from the liver as well as improving reproduction of red blood cells caused by liver malfunction. Potato Potato contains high amounts of steroidial saponins that helps to block the over-production of estrogen by increasing levels of phyto-estrogen, the element which is necessary for normalizing menstrual cycle.

I am in the same situation as you.. I am 5'0 and about 85 lbs, down from 119 in December. I haven't had my period in 9 months. I'm working on gaining weight, which hopefully will bring my period back. Adding back in some carbs after being low carb for a long time is helping- sweet potatoes, fruit, white rice... all are good especially when you are not looking to lose weight. My issue right now is that if I lose too much more or don't gain in a reasonable time frame, I could be forced to go on medical leave from school (I'm 19) which I really don't want to do.

The most important thing is to make sure your body is healthy and that means having a period! Good luck; I know where you're coming from!

I've actually been through what you're talking about. I started off overweight, lost a ton of weight, was 105lbs. at 5'6'', and stopped menstruating for a year. Honestly, I had to gain back to 115 by eating what I thought was a ridiculous enormous amount of food. It only seemed like a lot of food because I wasn't used to the increased volume of whole foods - so in essence I got fuller sooner eating paleo (as opposed to eating grains/processed food). HOWEVER, I DID get rid of a lot of health problems I had, such as crappy menstrual cycles, acne, etc. So it wasn't all bad times, but I think that the reason those things disappeared was because I wasn't eating crap food. But the periods went away because I wasn't eating ENOUGH food in general. So, eat more. Eat more meat, eat more fat. It has nothing to do with carbs. I eat under 70g of carbs per day (I measure my food - this is not overestimating!) and I have lots of energy and regular cycles.

For the lack of menstruation, consult a holistic women's practitioner. I would personally recommend Evening Primrose Oil -- as a fairly lean female myself, I too have had no periods for an extended amount of time. Eating meat after being a vegetarian and vegan for a while helped A LOT, but after Paleo, I also added in the Evening Primrose which has really helped in keeping my periods regular.

http://www.tccah.com/amenorrhea.htm

You can probably eat a bit more of the carbs if you are not interested in losing any more weight. More info on amounts here:

http://www.marksdailyapple.com/the-primal-carbohydrate-continuum/

Even if you don't want to gain much more weight, make sure you are steady with good protein sources and fair amounts of saturated fats.

Good Luck!