doctors physical tomorrow /doctors questions.....

I have a physical tomorrow and i don't know what to say if she ask me if I'm sexually active... like yeah I am but I don't want my mom to find out . I am 18 and my boyfriend says I should tell my doctor because he did when he had his but it's different  for guys. I don't know what to do and my physical is tomorrow afternoon 🙄 my mom is dropping me off so she won't be in the room but what will my doctor do next if I tell her?