I think you just need to talk to her about it. Try to avoid a fight over it, but you seriously need to sit her down and find out what FOR SURE is going on there with her other guy friend.
Another thing, it doesn't sound like she is as into your "relationship" as much as you are. Find out...ask her exactly how she feels....
I have had my girlfriend now for 3 1/2 years!!! That's a long ******* time for only being 17! And she's 16! We've definitely had our down times, but we've also had many more great times than the bad. We have broken up twice since we first got together. The first time we were apart for about a month, we both tried to hook up with other people, and it wasn't very cool. So we talked and got back together. The 2nd time we were in a fight and I blew up and stupidly broke up with her again. We were back together in a week
....
What I have realized with my girlfriend though, is that my life, when we are dating is so much better than when we break up, even though I broke up with her both times.
3 1/2 years is a lot different than 5 months though....so talk to her....demand the truth from her....and react....do what you feel is right......all I can say to you....
And good luck!