Sorry I messed up the title** I meant choosing her friend to go with her to prom instead of me

I’m new to these, but me (bf) has been dating (gf) for more than a year. We had brought up the idea of going to prom or senior ball for a while. Since we began dating we mentioned it and would talk about it. Slowly as time went on she seemed uninterested about it and didn’t want to go but earlier today she said she wanted to go and was going to buy her ticket. but she wanted to go with her friend instead. I felt bad because I had been looking forward to it for a long time but also they have only been friends for a very short amount of time. I felt upset that she chose her friend over me but AITA for being upset at her?