So my girlfriend of 3 months has been going to a friend's place for him to help her out with a programming assignment for university (I have the same assignment), a couple of times in the past 3 weeks.
I'm an open minded guy so I was ok with this for the most part, I know how tough the assignment is and she's not quite as proficient as I am + we have a really tight deadline with lots of other work to do. She lives out of town and he lives in the same town as me, I've never seen him, but she always told me when she was gonna go.
Today when she called me to chat before she took the train I decided to surprise her and head to the station to meet up with her, maybe help her carry the laptop, you know, just to see her. As I'm walking towards the station I see her, a guy next to her, carrying her stuff. She sees me and looks more than a little startled, I walk across the street to meet up.
She says 'I was just thinking about you' with a big awkward smile, I say 'Really? Well, I called you like 3 times and sent you a message...', she says 'Oh I didn't see!'. I'm thinking, ok, that's possible after all, her cell's speaker is busted, she just has it set to vibrate and it's always in her handbag. Then she goes on to ask me how come I came, why I'm not working on the assignment etc. all the while she's obviously uncomfortable and unnerved. I'm thinking what the hell, I'm not exactly comfortable with this whole idea either, but I don't want to be a control freak, so I walk and talk with them part of the way and then the feeling of utter misery takes over completely and I tell her I'll hear her later and say goodbye, she says bye, they walk on, I walk home. All the time I'm thinking 'Damn, that guy seems VERY familiar...' The minute I got home I got a message from her saying 'Hey kitten, I'm so sorry

I didn't even see the calls or messages'
As I got home I was trying to remember why he looked so familiar... and then it hits me like a freight train - a year ago I saw the two of them kissing in the university lobby. Needless to say, I've been feeling a mixture of blind rage and complete misery, only compounded by the fact that when I got home and turned on my computer I saw she was signed in to msn and idle. She stayed idle for 3 hours, now I'm not one to be paranoid, but that's a little bit odd considering he was supposed to be helping her with the assignment.
Half an hour ago I get an SMS on my cell, she says 'What are you doing?', then five minutes later before I even got a chance to write up an answer I get another one 'You're not answering

'. Now that looks like a sign of desperation, maybe fueled by guilt? I answer that I'm working on the project, ask her how it's going over there. She answers back that she's working too (with a '

' again) and asks me whether I would be up for meeting her later, in two hours. I tell her 'Sure, ring me up when you're done'.
Here's the deal, I feel miserable, kinda betrayed but I'm faking that I'm ok with it through my messages, after all I could be overreacting. But if I do meet up with her I don't think I'll be able to hide how angry and disappointed I am.
Any thoughts, advice, anything?
BTW, I'm 23, She's 22 and he's 26 or so.