Is it possible that my GF imagined me kissing another girl at a party?

Keep telling her that nothing happened. She may have dreamed something, or misinterpreted something she saw, but she is convinced that she saw something, and nothing you are going to say will change that. If you keep insisting, she may figure that you don't remember it. You could also ask friends that you were with if they saw you kissing another girl. If she can't find anyone else to corroborate what she saw, maybe she'll start to doubt herself.