What exactly causes period pain

 
During the menstrual cycle, the inner lining of the uterus, called the endometrium, thickens in preparation for a possible pregnancy. In the event that a fertilised egg does not implant in the endometrium, it sheds off at the end of the cycle. This process of shedding involves constriction of blood vessels and some amount of tissue death. It is this process of shedding that causes pain. Some people are more sensitive to this pain than others.

There are other reasons why this pain may be severe. Presence of uterine fibroids, endometrial polyps, a condition called endometriosis etc can cause severe pain. Any person complaining of period pains must be evaluated to exclude any of these conditions.