Why do cramps hurt soooo bad sometimes?

Hey I have had my period about 4 years and sometimes I get cramps so bad that it hurts to walk my back and thighs hurt and I throw up and get sick to my stomach. When my cramps get this bad Pamprin Midol Advil any pain killers don't really work to get rid of the pain. And other times during my period my cramps are very minimal to non. But that doesn't happen often. So why do cramps hurt so f ing bad sometimes?