shes cheating on you

seriously, if you have no reason to not trust her... there is nothing to worry about...
EDIT:
If she flirts back, who cares? shes not banging the dude or whatever... getting pissed over that will just lead to fights
not that i'm saying let her do whatever, but don't do it constantly...

I find it girls like being told what to do sometime...just dont be a pig about it