Sex and Telling

I am more closer to my stepmom than my biological mom. I had sex and I want to tell my stepmom, but in worried she will tell my mom. If I told my mom I had sex she will freak. When I was in 7th grade my stepmom told me if I was having sex let her know so she can put me on birth control and more protection. I want to tell her but I don't know what to do. Someone please help!! Btw I'm 16 and a Junior in hs