Well here is why I think girls cramp on their period because their endometrial lining is thinning and that is why there is blood. Menstrual cramps are called dysmenorrhea. The cramps are caused by contractions in the uterus. The uterus is a hallow, pear-shaped organ that the baby grows in. If the uterus contracts too strongly it can press against the blood vessels cutting off supply of oxygen to the mussel tissue of the uterus. Pain results when a part of a muscle briefly loses its supply of oxygen.