Thank you so much for your reply and advice.
My girlfriend and I discussed all of this at length and sadly I learned it hasn't been just one guy. The guy she met the night I showed up at her apartment is one she's been having an affair with for the past 2 months. She claims to have had a one-night-stand with him about a year ago and then they started hooking up earlier this year. She also told me that she had been unfaithful on another occasion…both men are co-workers.
Obviously, I've completely lost trust in her and I'm deeply hurt by the discovery. She cried a lot (well, we both cried a lot) and she told me how much she loves me and wants to work things out but I think its better for my own mental health to move on. It pains me to realize this but I don't think she will change and I want a happy, fulfilling relationship with someone who is faithful and trustworthy.
Again, thank you so much for the advice.