Why does my mom ume the worse?

What is her problem?! Yesterday my boyfriend came over when no one was at home but me since my mom, step dad and siblings were all at church (and I have to admit that I lied to get out of going since I didn't feel like dealing with it). Keep in mind that I didn't invite him over he just came over on his own since he needed to talk to me about this problem he's having with his dad but anyway when we are in my room my mom just bursts in out of nowhere and asks him to leave! And we weren't doing anything! Even though we were lying on the bed but we were just snuggling, honestly! But she umes that we must have been having since the door was closed and we were lying on my bed and when I tried to tell her otherwise she wouldn't believe me and said if he really wanted to talk, we could have done that on the front porch since I'm not supposed to let anyone in when she's not there! She wouldn't even talk to me today. We are out of school today but she told me to come to work with her to run errands in her office since I obviously can't be trusted to be home alone! I don't get her deal, since we really weren't doing anything and we never have anyway! Why would she ume something like that? What is with her?