Birth control, but don't know what my mom wil say.?

I have a really irregular period. I've had my period for 4+ years but it still is always too late or too early, and it's really annoying. I am always trying to figure out when I get it and I'm always wrong and it really frustrates me so, a choice to regulate my period? Birth control. I'm really afraid that my mom will think that I'm having , which I am not. I don't want her to think that I'm promiscuous, it's not the reason why I want birth control. I'm 16, almost 17, but that doesn't really mean anything. And I don't want to go behind her back and get it on my own. Help me with a conversation starter or some way to encourage her to allow me to get it, thanks!