I'm upset with my girl because she thinks I'm cheating
Does she have a reason to think you're cheating?
According to her I haven't been acting the same as before.
I can understand why you're upset that she thinks you're cheating even though you aren't. I think you should talk to her about how you're faithful to her, and about how you're hurt that she doesn't trust you.
I don't want to because she makes me sounds bad about it. I'm going to make her feels bad about it and have fun.
While it is reasonable to be upset with your partner if they think you're cheating, but if you are not then you should explain that to your partner. Let them know everything that has been going on.