Why do you have menstrual cramps?

Prostaglandins: During the period, a physiologic process occurs which leads to the sloughing (shedding) of the inner lining of the uterus, known as the endometrium. What facilitates this involves the production of very powerful chemicals by the body known as prostaglandins. These produce very strong contractions of the uterus which in some women can be as painful as labor. This also explains menstrual diarrhea.