Why is she such a bitch?

Tonight my girlfriend and I had a date night. I took her to get her nails done then we went out for dinner. We came back to her place (which her roommate knew about) and started having a few drinks while watching a movie in the living room. As I started training to make a move we decided to move to her bedroom. That's when her roommate told me I couldn't spend the night, because she didn't want me there. She even tried calling the cops on me. She ruined our evening. She won't stop being annoying. Why is she being a bitch.
Why is she such a bitch?
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