<human>: Provide a brief overview of the text.

Reddit,

I have been in a two week exchange program in a foreing country with my girlfriend. There, we made a lot of friends, but one of them seems to be a bit more for my girlfriend for 2 years, whom I share my flat with.

Yesterday, around a week after coming back, I started my laptop to prepare it for work next day, and her facebook was still logged in. It was around 2 am. I didn't notice instantly, as I had chrome on background, but suddenly chat sounds started coming and I watched a conversation unfold between her and this guy. It was, I guess, an ongoing chat, as they started speaking very intimately. She was really pushing for it, sad about nothing happening back there between them, and flirting and teasing him, in the end she even told him if she wasn't with me, she would be with him. Then adding I should never know about that. The chat went on up to 4am.

I am always 100% loyal and I have always given my best in the relationship, so I don't tolerate this kind of stuff, but the guy lives in another country and it's impossible for them to meet.

Subtlely trying to get her to talk to me about it gave no results, she acts as if nothing has happened.

What do I do now? I value her and love her a lot, but right now I feel like I don't really know her, being so different her words towards me and what she did behind my back. Should I bring it up? Any similar experiences?
<bot>: GF for 2 years flirts in fb chat with guy we met in an exchange, said she would be with him if not for me.