telling my mom i lost my virginity

my mother at one point wants to bring me to the gynecologist and i had sex once and she wants me to tell her when I become sexually active but I'm still 15 and I don't think she will be happy to learn that I lost my virginity at 15 and never told her. How do i tell her i am sexually active. (when we had sex we had a condom and he pulled out and I had my period since then)