Every 28 days (on average), if a woman’s egg is not fertilized, the uterine lining will be shed as menstruation. Menstrual cramps are caused by a hormone-like substance called prostaglandin, which occurs in the uterine lining in between menstrual cycles. Prostaglandin causes the uterus to contract, just as it would during labor. Furthermore, it can cause nausea and diarrhea. The more prostaglandin is produced, the more severe the symptoms, including uterine contractions, which increases the pain felt.