How Do I Tell Her the Truth?

I'm 16 and about 2 years ago I started having sex. I visit my friend a lot (who's a girl) and when I visit her house for hours and don't come back home until night, my mom is always suspicious but she's never said anything until I got vaginitis (yeast infection) but not from sex. She asked me once or twice before this but every time she does, I tell her no, she asked me: "Are you having sex?","Have you been with anyone?" - and I told her no, each and every time she asks. My oldest sister is also sexually active and even got pregnant but my mother never found out that she was sexually active or pregnant. Now, since I have the yeast infection, my mother decides that she's going to take me to the doctor.. My question is, since the doctor is going to ask me questions about sexual activity, how do I let her know that I have been sexually active in the past 2 or 3 years before I get to the doctor and she finds out that I have indeed been sexually active? (I've never had an STD or VD or pregnancy's or anything I'm REALLY safe when it comes to sex and I make sure that my partners are also)