telling my doctor I'm sexually active-15?

I want to start taking BC pills to help regulate my periods and prevent pregnancy.  I've only had sex once and it was with a condom, I obviously haven't told my mom, but I told her I want to go on it for my periods. She's going to be in the room with me when I go to the doctor, so I can't say that I'm sexually active in front of her, but I want him to know that it's not just for my periods so he an give me something that will help keep us (my boyfriend and I) protected too! How do I go about this?