Sorry you haven't gotten any replies yet. This is a tricky one.
I couldn't tell for sure from your post if you are actually having sex with your boyfriend or not. It sounds like it, and if you are having sex with your boyfriend then THAT is much more important than your periods and that is why you should be going on the pill (I assume you also don't want to get pregnant, not just ease your period pain).
If you're having sex with your boyfriend, you have to be mature enough to talk to your mom about getting on the pill. If you want to go the 'it's for my periods' route, give it a try. She might be happy living in denial
And if you really can't do that, try Planned Parenthood or look for another clinic in your area.