Should my mom be worrying about me having sex at eighteen years old?

I have been together with my boyfriend for over four months. I am 18, he's 19. Last night I slept with my boyfriend and my mom found out. She was not happy at all and called me a slut. i told her we used protection, but she is still not having it. Should she be worrying about my sex life when I am an adult? Why is she trying to make me feel guilty like I did something wrong?