I need birth control, but how do I tell my mom?

My mom and I have a rough relationship, but she's the one who has custody of me. I understand that I don't need her there to get birth control, but I would need my medical card as well as a ride. My boyfriend and I have had sex once, and we're both scared to do it again because we don't want a child. Yes, we used a condom and we will never not use one. But there is still that chance we don't want to take. He is 19 and I am 16. Me staying at his house and us being together is parent consent. My mom loves him. I just don't want her to not trust me. I'm extremely safe and I've changed into a way more responsible kid. I'm just trying to do the right thing if I'm going to be having sex.

Any suggestions as to what I should do?