Menstrual cramps?

Menstrual cramps are pretty common — in fact, more than half of all women who menstruate say they have cramps during the first few days of their periods. Doctors think that cramps are caused by prostaglandin, a chemical that causes the muscles of the uterus to contract.
Depending on the girl, menstrual cramps can be dull and achy or sharp and intense, and they can sometimes be felt in the back as well as the abdomen. These cramps often become less uncomfortable and sometimes even disappear completely as a girl gets older.
Many girls and women find that over-the-counter pain medications (like acetaminophen or ibuprofen) can relieve cramps, as can taking a warm bath or applying a warm heating pad to the lower abdomen. Exercising regularly throughout the monthly cycle may help lessen cramps, too.