Period cramps hurt because of a chemical called prostaglandin, according to the Cleveland Clinic. During the period, this chemical causes stronger uterine contractions. This, in turn, restricts the supply of oxygen to nearby blood vessels, causing pain.
Painful cramps can be a normal part of the menstrual cycle. Unfortunately, one out of 10 women suffer from pain so severe that it impairs normal daily activities, according to the U.S. National Library of Medicine. The Cleveland Clinic reports that conditions such as infections, uterine fibroids and endometriosis may cause severe period pain. In these cases, women who suffer from severe pain during their period should be under the care of a physician.