not sure if i trust her

So i've been dating this girl now officially for about 7 months nows im 23 and she is 17 now im not here about the age difference and really dont want to hear about it. The parents are ok with it and in florida its legal, my issue has nothing to do with it. Problem is about 2 months after we had started dating i happened to be on her facebook cause she left it open on my computer and i saw a message between her and another guy where she was talking to him and asking him if he would like a **** buddy. Shes never met this guy before and when i confronted her about she said it was all just a joke and she was playing around with him and would never cheat on me. ofcourse after seeing this i lost trust in her. most of her friends are guys and since that day now i get really jealous about any of her guy friends and i dont trust her being around guys. We've talked about it several times and she apologizes about the situation and says it will never happen again and assures me that she will never cheat on me. she says she loves me alot and has recently done things to show her love, like she surpriised me on valentines day by waiting for me at my house in my room filled with candles and rose pedals. The other night she also surprised me by coming out to this club i DJ at un announced. I love this girl but ever since seeing that message i always have doubts in my head, and she has since let me see her text messages on her phone and i've seen recent facebook messages where guys have asked her to hang out and she tells them that she has a boyfriend who she loves very much. Do you think she really was just messing around with this guy and playing a prank on him or could she really be doing things behind my back. For the most part she tells me where she's at all the time and the people she's going to be with but i always feel as if she could still be lying to me. please help.

Also forgot to mention that she cheated on her previous boy friend to be with me which also worries me because if she has cheated before who to say she wont cheat again.

The thing about relationships is that they are all about trust. This is a difficult thing, but we must learn in our lives to trust even if we have been burnt before. It sounds like to me she could have been messing around with the guy on facebook. You mentioned that she has many guy friends, and the way that girl and guy friends interact can be a bit different than expected. I personally have mostly guy friends and we joke about many things, and are really up front about things. So it could have been a joke, but even if it wasn't if you want to be in a relationship with this girl you will have to give her the benefit of the doubt and offer her trust. The more you make a big deal about this, and the less you trust her the more she will pull away from you, and be tempted to cheat. I can say this from personal experience. It can get to the point where its almost worth saying "I might as well do something with another man, because he thinks I am anyway."
Most women do not want to a micro manged or doubted and if this is happening it will put walls up in the relationship. The closer and more open you are in your relationship the less you need to worry. She probably does love you very much, and if her needs are being fulfilled then there is nothing to worry about.
So remember in a relationship you always mirror each other, and if you are displaying trust, honesty, and respect, than she is more likely to as well. Good luck, and may your relationship be happy.

That having been said ... if she cheated on her last boyfriend to be with you ... keep your eyes open!

Dont be stupid man, ask yourself, for that reason should she "joking" like that with other guys. It's normal - she don't want to loose you, so she tries to fix the situation. But dispite that, don't forget about other guy she had dumped ;)

Thanks everyone for the response, im trying to trust her and like "shasta" said i think it was in a joking way. Only problem is im a jealous guy really jealous guy and she has a lot of guy friends and i've tried telling her several times that guys are only after one thing and you cant really trust them. I have kept my eyes open and been on her facebook with out her knowing and seen where she tells people she's never cheated on me. for some reason how ever i cant get this doubt out of my head or the jealousy. I mean just seeing her hug a friend of hers she runs into the mall makes me want to punch the guys face i dont how to overcome it either and she's starting to get very fustrated about it too. Its driving me crazy and frustrating her and causing us to always get into arguments about it too. It seems when im with her how ever everything is fine because i know she's with me and what she's doing but when were not spending time together i always wonder who she's with and what she's doing with them. Like on of her best friends she hangs out with all the time is a guy and she says they play video games and what not but for some reason i always come up with a bad scenario that they are doing alot more than just that. I need help guys.