HOW SHOULD I TELL MY MOM

Guys, how would you tell your freaky mom that you had sex with your boyfriend after 8 months of dating without making her get overprotective and super unnecessarily worried I might get pregnant anytime soon? I mean, I feel like she'll get really annoying about it, so I haven't told her yet. Sometimes I feel like I don't really have to tell her, I'm not doing anything wrong, we're protecting ourselves correctly and all, but I'm afraid she won't see it that way. She still sees me as if I was 10, even though I'm 18 now. We have talked about me having sex a few times like it was not really happening ('cause she doesn't know yet) and she looked like she wouldn't care that much, but sometimes it felt like I'd be the worst daughter ever.