Talking to my mum

My mum wants to know when I have had sex but I still haven't told her, I'm going with her to see the doctor about birth control etc in the week. However things are patchy at best with my boy friend and I don't know how to bring up the fact I've slept with him if it's likely we will break up soon. I don't want her to be mad and I want an implant cause even if things don't work out with him I will continue to see other people and don't want to have discuss this with her again. Any advice would be welcome, thanks