is this possible??? gf cheating?? hackers or computer savvy please help!!?

[Tom]

Me and my girlfriend have been dating for a year now. i just recently looked at her facebook and there are chat messages in her inbox between her and her ex saying they wanna get married, shes cheating, and how in love they are with each other there are also missed and received video chat notifications from both parties. when i confronted her she denied it and said she hasnt contacted him. normally i wouldnt believe this, but she has always gone out of her way to treat me right says she wants to marry me and she has him blocked from every social site possible. she even has his number blocked from calling her phone. he was extremely psycho and harassing to her for the first 4 months we were dating. it doesnt make sense to me because ive known her for 5 years before we dated and shes never been one to sleep around or cheat. my question is are there any hackers or anyone out there that knows if its possible for someone to plant fake conversations with someone in that persons actual inbox not just show a picture of it?