<human>: We have been together for 3 years. For the past few months she's been acting distant. She's also started taking Saturday night shifts at work. I tried to tell myself that it was nothing, but it kept bugging me. Last night I decided to follow her as she left for "work." I got my friend to drive me so that she didn't recognize the car. Anyway, long story short, I saw her go to a strange house, kiss the guy who answered the door and then enter the house. Probably the worst part is that this is literally the first time that I followed her, so how many times has she done it before?

I'm at college now but usually she would get home from work an hour or so ago. I am assuming she's there now, I had an early lab today so I had to leave early. 

I don't know what to do. I haven't spoken to her since she left last night. Do I just go home later today and tell her I saw her? Do I just throw her stuff out of my apartment? I know I sound stupid but this is my first relationship so I've obviously never dealt with infidelity. I've always had trouble feeling my emotions, especially in times like these. So I've generally sought comfort in rational sequences of steps in which I could follow. I just need some idea of what to do at this point.

Can you give me the gist of the text in a nutshell?
<bot>: I [M21] followed my girlfriend [F21] of 3 years to another guys house while she was supposed to be working. What do I do now? What do I say to her?