First of all, congratulations on getting your period back! Amenorrhea (loss of period) has serious side effects, including loss in bone density, and of course, due to lack of ovulation, it is a form of infertility. I am currently being treated for amenorrhea in Finland.
Amenorrhea can be caused by low body weight, over-exercise, stress, disordered eating, as well as by medical conditions such as thyroid problems and poly-cystic ovaries. When it is due to energy deficits, whether through over-training or under-eating, or when it is due to stress, it is usually Hypothalamic Amenorrhea. This can happen even if your BMI is in the normal range, and their are many women whose BMI is low, and yet do not suffer from this. From the research I have read, BMI really isn't the best indicator. Every woman's body is different.
I would say that the weight gain is a necessary part of your healing. The article [Overtraining the Female Athlete] recommends:
- Decrease training intensity/duration by 10%
- Attempt to increase body weight by 2kg
- Supplement calcium intake to 1500 mg per day
- Gain psychological support
- If amenorrhea is persistent after 3 months of modifying behaviour, gain medical
While this is regarding amenorrhea due in part to over-training, it seems to match with other regimes I have heard regarding recovering from Hypothalamic amenorrhea: I have heard weight gain is almost necessary. Also, it might be good to question if stress or similar had a part to play, and whether that still needs to be addressed
A month ago I added whole dairy and fish back into my diet, and for the last week and a half I have replaced running with walking and yoga. I have seen an improvement in my hormone levels. It was a STRUGGLE to add dairy and fish back into my diet, too-- and to stop running. However, it is also important to remember that amenorrhea can cause bone loss. Diary should help with calcium intake, and FULL-FAT dairy consumption seems to promote ovulation and natural, fertile, cycles.
So, i would guess that the dairy and weight gain are important factors in your recovery. Your body needed, and needs, the excess energy to restore reproductive function. The important thing give yourself time to heal completely, and to consider the menstrual cycle as a important vital sign for the future. The need for increased weight might be a temporary phase needed for healing, but it might also be good to explore seeing the beauty in your still VERY slim but now FERTILE shape.
Amenorrhea is also discussed, helpfully I think, in the book Taking Charge of Your Fertility by Toni Weschler.
Best wishes for a continued recovery!