Need advice about a situation with my gf

My girlfriend (no pics) and I have been together for a few months and today she told me that an old friend from college is going to be staying with her and her roommate at their place for the next few months because he’s going to be in town for work. I was a little taken aback at first, but then when I asked her more about this guy, she tells me that they did hook up several years back in college, but it was only once. I feel very uncomfortable about the entire situation and I want to know what y’all think I should do or what would be the best way to handle this. I wish this was a troll