Why do you think she did that?

My so called "best friend" is a girl named jessica. today in band cl she was acting really weird to me, i seriously had no clue why. Later on I found out that Jessica (my friend) told the girl whom i will be taking to my schools homecomming, not to go the the dance with me. My date asked me why jessica tol her that and i really dont know why. any suggestions of what to do in this scenario? Im realle Pissed so Id really appriciate all of your answers. Thankyou (Oh keep in mind i took my friend jessica to homecomming last year, and she said she had a good time, but why would she tell my date that this year?) Jessica has a boyfriend also keep that in mind.