Cramps, need help!

I got my period again but it says I don’t get it for another 21 days? My cramps are excruciating! I’m confused bc i just got off my period.

I seriously want to go on birth control because my cramps are bad every time I get my period. I passed out last year at school because of them. But my mom wont let me go on it because she says I am too young (I am 15) and most of the girls I know use it for those reasons. She thinks I’m going on it for sex. Not knowing it helps, when I tell her it does, she says it’s bullshit and I should just take ibuprofen. Which doesn’t even help. Any ideas for medicine to help cramps?