Why was my Dad's girlfriend being so rude to me?

My father, a single parent, has a girlfriend now. They've been together for over two years. She was really nice at first, but last night, my dad wanted me to go out with them because he doesn't like it when I'm home alone a lot. So I go out with them. When we drivew up to her house, my dad wants me to go in with him. So I make him happy and go in with him. A while ago I had back surgery to help my posture. I'm better now, but because I was used to slouching all the time, my brain needs to catch up with my body still. When I saw her she said I look like I never had any surgery, and she meant it to be mean! My father even supported her! I might be being sensitive here, but why all of the sudden is she being rude to me? That wasn't the ONLY rude thing she's said to me. Opinions and advice on how to handle the situation please?