Cramps, every woman's worst enemy. Or one of them, at least. Here is kaytay's Cramp Scale:

1 - 3 : A slight yet almost unnoticeable discomfort in the lower abdominal area. Usually able to ignore.
3 - 5 : Still just a pain, but becoming more serious. Unable to ignore anymore. A few Advil should take care of them. Saltine crackers are recommended.
5 - 9 : You get a permanent grimace on your face and walk hunched over, clutching your stomach and moaning like there's no tomorrow.
10 : PAIN. There is no other word. It feels like a punch in the stomach, like your uterus is trying to claw it's way out of your body and then attack you some more. Usually involves frequent trips to the bathroom when you are not completely immobile and miserable. These are one of the only times when you'd give anything to be a guy.