I need help with what to do about my girl friend?

Okay so i was at a Military camp for one week last week. No phone no computer. No contact with my girl friend. So i get back and everything is fine and normal until last night i ask her if she hung out with any guys and shes goes "yeah i had a bagel with a friend" and i say "who?" And she says this guys name that she had a thing with and did stuff with before me and her started dating 5 months ago.. So i was just kinda like umm what the hell? then today we are talking about it and i ask her if they hung out after he picked her up (it was just the two of them) and she said yeah he watched me do the dishes and then left and i was like okay... well anything else you want to add on to the story? and she says we watched tv for a hour or so and talked and cought up on things. I feel really weird about it. she has been saying sorry and **** all day and crying her eyes out. What should i do?? She said nothing happend so what should i do? She is telling me she loves me and would never do something like that