How do I tell her this??

So me and my boyfriend have recently started having having sex, and my mom told me that she wants me to tell her if I’m having sex so she can get me condoms and put me back on birth control, (We are using condoms and I am going back on the pill to help with my period pains). I don’t know how to tell her without her getting mad at me because she also always says that she doesn’t want me having sex at this young (I’m 14) I don’t want to make her mad but also I don’t want to have to sneak around anymore. Any advice on how to tell her or what I should do?