asking parents about birth control

i have really bad periods and i just want birth control to help with that, but if i ask my mom i know she'll get the wrong idea. we've always been really open with each other but just the other day she told me she was concerned that my boyfriend and i were being "inappropriate" and "too comfortable" when she saw us napping together because i was laying on top of him. she also told me she doesnt want me having sex with him or being 16 and pregnant. i dont want birth control for sex especially concidering im turning sixteen in a couple months and im not ready for that yet, but i really just want it for my periods. and maybe in the next year i will need it for sex but im not planning on having sex right now. i just need some advice on how to ask her without her getting the wrong idea.