I feel like I can't tell my mom anything

ok so I am 15 and I've had a boyfriend for almost a year and things are getting serious but I want to wait to have sex and hes totally ok with that but my mom has been asking me questions about sex which is fine but I asked her that if I did decide to have sex would it be ok if I went on birth control and she said yes and then she went outside and told my step dad that I wanted to go on birth control. I didn't want her to tell my step dad I just want it to be between me and her and I feel like I can't talk to her about this kinda stuff Cuz Im worried that she will tell my step dad. Should I still talk to her about it?