How to tell my mom I’ve had sex!(HELP)

So my mom told me that when I have sex to tell her so I can get birth control. She scares me though because she will be nice and “supportive” like that but then she’ll ask a bunch of questions like who, what, where, and why did I do it. Then if we ever argue she will bring it up as an insult in some way. I’m 15 and I don’t know how to open up to her about these things. I always feel threatened by her in a way!