The frequent diarrhea that accompanies menstruation for many women. Prostaglandins are released during menstruation causing the uterus to contract and cause cramping. They also speed up the contraction of the intestines, hence large amounts of crampy, watery stool. Very similar to the hershey squirts, though period shits occur only during menstruation.
I thought I was in labor, but then I realized it was that time of the month so I went to the bathroom and had period shits.