Definition: Given a real-life anecdote of a complex ethical situation, generate a suitable title that describes the main event/root cause of the situation. Imagine that the given text is a social media post, and you have to write the title of the post, so the users can decide to read the main text or not.
Input: (Background: our school has a rule that undreclassmen can go to prom, but only if a senior takes them)
So this story starts back in January of this year. My gf and I had been together for a few months and prom was coming up. She had never properly been asked to a dance before so I wanted it to be special. I spent about three hours making a poster with the word "prom?" On it in several different color patterns. She happily agrees to go with me. Everything is fine until a few days ago, when my gf and I were walking together. A mutual friend of ours stops us and tells my gf that she got the go ahead from our VP and all they'd have to do is get their prom tickets. When I inquired wtf she was talking about, gf chimes in saying she was taking other girl to prom so she could go (at least a month after she already agreed to go with me). When I asked her when she was planning on telling me, she responded that she had been meaning to, but hadn't since she figured I wouldn't mind. After that I walked away and didn't hear any more of it. 
(Not worried about her cheating on me or anything, just upset that she agreed to this without telling me and assumed I wouldn't mind)
Is this no big deal? Am I overreacting? I just need some unbiased opinions on this situation. Also this is my first post so I'm sorry if I do something wrong. 

being mad @ my gf for agreeing to take another girl to prom