Is my Girlfriend cheating on me?

but truth is my girlfriend’s friend really like me and wants to be with me shes sorta gave me clues before but she said that my girl friends a bad choice and she seen her recently flirting with this kid and shes playing with me. And also my girlfriend is going to prom with some kid from her school that’s a senior my girlfriend is a sophomore but to my prom shes wearing a backup and when i told my gf today if everything ive been hearing is true she thought i was crazy and started crying when i explained and her friend ask what i told her and i told her and she was so happy when i said she was bummed and stuff but who should i believe cause she tried stuff like this before and the first time she did me and my gf hung out right after and had our first make out for like a whole hour help me please who should i believe?

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