I need advice

My boyfriend went down on me on Thursday, And today is Saturday. I want to tell my mom I want to be put on birth control just in case if something happens, because you never know if something is gonna happen. But how do I tell her without her knowing me and my boyfriend do some “Freaky” stuff. (Fingering me, jerking him off, going down on me, and me going down on him.) I don’t want my mom to know because she won’t trust me. She started doing the same stuff at my age, she had sex when she was 16, I am 15 (please don’t comment about me being “too young” im fine I promise) and that’s when she started doing things like this. She had me at 17, I’m 15 she’s 32. I don’t want her to know the things I do because she won’t trust me and him alone anymore... so how do I tell her I want to be on birth control without her knowing me and him do things like that?