Menstrual cramps, Dysmenorrhea

Menstrual cramps (dysmenorrhea) are the cramping pains in the lower abdomen, usually on the first or second day of the menstrual cycle. This is caused by the contractions of the uterus. As the uterus contracts, it can press against nearby blood vessels, cutting off the supply of oxygen to the muscle tissue of the uterus. Pain results when part of a muscle briefly loses its supply of oxygen.
Severe menstrual cramps, particularly if paired with excessive bleeding or passage of large blood clots, can occasionally be a sign of endometriosis or other disorders of the female reproductive tract.