To be honest I personally believe I have trust issues because I rarely trust that any of my girlfriends are faithful even when they totally are. But here's my advice:
The way you describe the story it sounds like your girl is flirting with another dude. This does not necessarily mean she has the intention of cheating (though it may) it could also be a shit test to see how you will handle it. Laying down the law means nothing because if she wants to cheat she can. She is in control not you, unfortunately
The more important questions to ask yourself are these:
-How has the relationship been going? Does it have a spark or fire of something great?
-Do you feel the same way about her you used to?
-Have you guys been fighting a lot?
If the relationship is drying out it is likely she is looking to somebody new...this is the natural order of things. Either in a relationship you realize that you are with somebody you cant be without or you move on. That being said your best chance of fixing this issue is to make the relationship better which can be done by work on yourself. Dont force annoying talks with her. Dont insult or front on the guy. Just be a happy exciting guy, maybe do something spontaneous with her, hell do something exciting without her, make an incredible story out with your boys, get kicked out of a club, whatever suits your fancy. But also keep a watchful eye...you never know, and I hate to say this, but she may be cheating....
Stay Fiery
Check out my blog btw
fierypickup.blogspot.com