I was busy doing work last night so went to see my gf before she went out with her friends. I gave her a kiss and surprised her and she loved it. as soon as i left she text me saying, nice surprise

etc...
she never text me when i text her in the early hours.
she later came down and told me she only kissed some other guy last night, she was upset, didnt try explaining herself or anything... didnt ask for my forgivness, just said i wud never be able to trust her again.
her flat mate later told me that some guy walked out of her room at 11am that morning... i asked her about it and she says he also walked her home, and slept on her floor :/
obs to me that she fucked him.
i just dont get why? it was all going so so well, and this guy is more athletic than me, shed only seen him twice before and didnt know him. none of her flat mates even know him and she only ever goes out with them
i just dont get why she did it... she said she regretted it and cant take it back, she didnt even defend herself, my theory is that she knows that i wud never sleep with her by my own free will if i knew.... and if i ever found out id go nuts.
just why?