Mom thoughts on birth control

Ok so, here's the deal, I am 14 and recently I've been having really bad cramps and I've always had this unusually heavy flow and it SUCKS. I want to go on birth control to regulate them. If it were just that I'd ask my mom no problem. But I have a boyfriend and I fear she'll think I'm doing it to protect myself from getting pregnant.  (Which in the future that might also be its alternative use but not for a while). Of course I probably won't tell my mom that because she would not approve. So what are your thoughts. What do you think she'll think and how should I approach her on this subject.