The exact reasons why diarrhea occurs during your period aren't fully understood, but it is quite common and often tied to menstrual cramps. Believed to be at the root of the cause are prostaglandins, chemicals released during your period that allow the uterus, and thus the intestines, to contract.lso avoid caffeine and junk foods as both can cause diarrhea and worsen PMS. Of course, a heating pad, warm water bottle, or warm cloth across your abdominal area can also help relieve the pain brought on by period cramps.