Should I break up with her?

me and my girlfriend have been going out for a year and a.half and have been talking for about 2 years 4 months Im a senior and.shes a jounior and I.didnt want to.go.to junior prom cuz I thought it is stupid she wasent going to go but 2 days ago she decided to.go with this other guy that she says his date couldent go.so he asked her last min prom was tonight she has alo had.a joking relationshipon facebook with him for about a year I dont know if this is terms to.break up I.just feel hurt that she went with him knowing how much I hate him prior to.this and.still said yes.... Btw im using my friends accout im a guy not a girl ... Please help