What is the cause of severe menstrual cramps?

Every couple of months (used to be every month) I get very severe cramps. It's horrible. The pain is so bad all I can do is curl up in a ball on my bed...and yes, it's made me cry before lol. I'm 19 and this has been going on since I was about 13. I take 800 ml of Ib profin because that's all that helps...sometimes that still doesn't touch it. It feels like I'm being stabbed with a knife from the inside...I'm sure others have experienced this. But my question is why!? Is it just hereditary? Or can there be other causes? My mom doesn't get them like I do...