Today Popular Science covered an issue I didn't even know was... an issue. But apparently when women menstruate, they poop. More. A lot more. The answer as to why, as with most of life's inexplicable occurrences, is easily explained by science. So let's let SCIENCE take it from here:
The answer is prostaglandins. These are the chemical signals girls' and women's bodies make and send to the uterus to tell it to contract, thus expelling the uterine lining at the end of the menstrual cycle. Prostaglandins aren't super picky about whom they talk to, however. If the body sends enough of them to the uterus, some stray prostaglandins will make it over to the bowel, which is located nearby. There, they also tell the bowel to contract, thus expelling… you know.
Poop. She was saying that the bowels then poop. These chemicals don't stop there. They also can cause nausea. So basically getting your period is like eating Mexican. For 120 straight hours. File this under things I wish I hadn't known 20 minutes ago.