What Causes Cramps During Periods?

Hello. I'm 22 years old and I'm just trying to find out why my cramps are so bad. Sometimes I just drop to my knees because it's so painful. It doesnt happen every period, sometimes once or twice a period, and maybe 3 times a year It happens about a week before I start. I took midol as soon as I felt the cramps beginning (before it got worse). This was about 2 hours ago and the cramps have just gotten worse. I have held my stomach and rocked myself, I've had a heating pad on it. Nothing is helping. Please help!!