First Gyn app questions ¿?

So I'm going to the Gynecologist for the first time tomorrow to get birth control (I'm 17). Obviously she is going to ask me if I'm sexually active (which I am w/ my boyfriend).. My mom doesn't know that I've had sex and I don't plan on telling her anytime soon. she thinks I'm going on birth control to regulate my period and make my cramps and bleeding less. When the Gyn asks if I'm sexually active do I just say no or does she usually have my mom leave the room?