Primary dysmenorrhea is caused by excess release of prostaglandin, a chemical your body produces to assist in the shedding of the uterine lining. Prostaglandin causes contractions in the uterus, the greater amount of prostaglandin released by your body the stronger and more frequent the uterine contractions will be.
The contractions also can restrict blood flow to the uterus. The longer and more frequent the contraction the less blood that can get through the veins to nourish the reproductive area. Areas that receive restricted blood flow can suffer pain. Prostaglandin can also affect the nerves in the reproductive area causing them to be more sensitive to pain and discomfort.