How to tell your mom you could/are be sexually active

My mother from day one has harped on me about sex and having a boyfriend. Almost everything we talk about turns into a lecture about sex, how its wrong and she often judges people,my friends even, that have already had sex,I am in highschool and im not a virgin and she doesnt know. Iv had to lie at doctors appts. About being sexually active because my mom always comes with. I dont tell my mom because idk how to, shes the type of lady who will stay mad forever and i dont want to ruin the little relationship we have. Any help please if im forced to tell her someday?