mom is physco about sex🙄🙄

I'm 18 years old and I've been dating my boyfriend for almost 6 months. We're both going to the local community college so we both still live with our parents. My mom is Christian and conservative and is CONSTANTLY freaking out if we are having sex and interrogates me about it every other day. My boyfriend and I are having sex and I'm on birth control. My mom calls me when I'm out with him just to see what we're doing to make sure we're not having sex. She talks to me everyday about how I shouldn't and how I'm gonna get pregnant and ruin my future and that I'm too young. I just tell her I'm not doing it because I'm scared of the outcome. TOnight my boyfriends grandpa was in the Hosptial so we went to visit him and my mom called and I told her we were just about leaving and she started freaking out saying how she thinks me and boyfriend are sitting around in the parking lot having sex. The past two months this is all my mom talks to me about and she's getting so paranoid about it. I feel like if I tell her I am she won't let me see my boyfriend. Somebody please help me !!! I'm sick of her interrogations and her checking up to see what I'm doing. Someone please tell me how to handle this situation!!!