Telling my mom

So I'm 18 and my boyfriend and I are having sex, my mom always asks me if we are and I just don't feel like telling her bc I don't really want to deal with the awkwardness and I also don't feel like it's not her business. She told me she keeps finding the thermometer around the house and asked if I'm trying to get pregnant! I think it must have looked like I lied when I said no because I couldn't give her a reason why but I couldn't tell her the reason Because I was taking it to make sure I WASNT pregnant... I could've told her but I didn't want her to know I've had sex.... Should I just tell her to avoid her thinking I'm trying to become pregnant?!