Should I get birth control against my mom's wishes?

I've been bringing up for YEARS that I want birth control (I'm 17 now). I want it mainly for my period, because I have severe cramps and a very long, heavy flow. But I also want it in case my boyfriend and I have sex. I told her that I wanted it for my period, not the sex part. She just keeps telling me she doesn't think I need to be taking pills that mess with my hormones, since she apparently had a bad experience over 20 years ago when she took it... My pediatrician thinks I should go on it, but my mom hasn't made an appointment at an OB/GYN for me, two years later. My question is... do you think it's wrong/dangerous for me to go to somewhere like Planned Parenthood and get the birth control for free without telling my mom or having her permission? I want to be safe, and I'd really prefer to have lighter periods. But my mom just won't listen to me! Please help. Thanks in advance:)