I've been going out with my girlfriend for the past 11 months?

[Football]

Recently, I moved to go university and she moved to London. Now there was a number saved on her phone as a girl but in fact was a guy? She owned up to saving this guys number. Now I asked her not to make contact with this guy but she didnt want to listen. I text the guy and he started saying he had sex with her & put up my convo on social media about it & about how the girl (my girlfriend) had never told him about it. I've never met this guy before. i don't know who to belive as my girlfriend swore on her life that she didnt. She doesn't want me to make contact with this person, she said she'll Handel it and make sure he removes the picture. I told her to ring him infront of me but she refused because she didnt want to give him 'missed' calls. She said she's only seen this guy in her uni around 3 times, that's it. A day after this whole thing happened she said that we weren't working out? We're still together but confused.