Estrogens are stored in you fat. If you have been eating Paleo and burning fat stores, these estrogens can be released as well and have can have an impact on your menses.

If you have been consuming much more vitamin C than usual, you will store more iron in your body.

In studies:

orange juice (as a source of vitamin C) increased [iron] absorption by 85 percent.

The body gets rid of excess iron from your body during menstruation every month, so if your body has absorbed more iron during a particular month, your period will be heavier.

Conversely, taking aspirin inhibits iron absorption:

taking an aspirin a day to prevent heart attacks and strokes causes blood loss via the digestive tract on the order of about a tablespoon per day. This results in iron loss.

So, it doesn't follow that your periods should be heavier taking aspirin alone, but I found an article which warns that fish oil and warfarin taken together may cause heavy menstrual flow; aspirin has similar properties to warfarin (both are blood thinners).

So, I would say that either excess vitamin C consumption or the combination of aspirin and fish oil are the culprits in this case.

It might be your thyroid. Hypothyroidism is known to cause menstrual problems, usually causing too-heavy periods.

Unfortunately, one side-effect of Paleo can be to expose what would otherwise be "subclinical" hypothyroidism.

You might enjoy Diana Hsieh's posts about this at her blog.

I had a similar monster in December (enough so I went to the ob/gyn) after skipping October and a light November. In my case, it was probably weight loss combined with perimenopause that had my estrogen out of whack with my progesterone (too much of the former). My ob/gyn put me on three days of progesterone to try and get me back on track. The jury is still out on that one though!

possibly thyroid, possibly trace gluten exposure if you are intolerant

Thanks for asking this! I just had a monster one too and couldn't figure out why. After reading everyone's responses, I think its a combination of eating more salmon, having to take lots of aspirin for migraines, and loosing around 7 pounds all in the same month.

Taking a stab in the dark - is there ANY chance you could have been pregnant with a non-viable fetus and had a spontaneous abortion (pregnancy that ended before it "took")?

I am told that this occurs more often than most people would think, but I haven't a clue as to whether you've been "exposed," so I don't know whether this is even possible in your case. :-)

Re: Louisa's post: While vitamin C would increase iron absorption, it wouldn't be a substantial amount of difference in that short period of time unless you were taking heavy doses of both that and an iron supplement every day. Even if your hgb and ferritin levels shot way up, the body doesn't actively use menstruation as an iron-clearing mechanism, it uses it to purge the uterine lining. You lose iron any time you lose blood, but you're not going to lose more blood if you have more iron.

In fact, vitamin C supplementation is a great way to prevent the anemia that is so common among premenopausal women. It's been shown to be more effective than iron supplementation. Combining both is the most effective route, though a copper deficiency could interfere as well. I think everyone should monitor their hgb and ferritin and supplement if they find themselves to be anemic.