This. You can second guess yourself and get your knickers into a twist for eternity, but you'll never know unless you ask them. Have you mentioned that it makes you uncomfortable, them always hanging out alone and talking after you go to sleep? Especially if there's nothing going on she should be receptive to the idea of cutting it back a bit. Relationships are about compromise. I know that my boyfriend wouldn't like it if I acted like that!

I think that you should talk to her alone, and not in the bedroom. Be calm about the situation and don't accuse her of anything!

You have a choice as was mentioned by other posters. You can torture yourself or you can mend the rift between you. No fight is worth the pain you are in. Now if you confront the two you are probably going to get lied to or your heart broken. I would suggest you sit down and calmly apologize for letting things go on like they have been. Let her talk to you about what she's feeling and just listen....HEAR her without trying to defend yourself or refute her logic. You will learn something about yourself if you do!
Then ask for the same consideration. She may not be able to give you more than a few minutes before she defends herself. Thank her for however long she gives you and let her rant. Don't internalize what you hear if you can, just listen...you will learn something about her and her past if you do.

Keep working on talking with her and you may find she breaks off anything she is doing with someone else without you ever needing to face that heartache. It matters what happens in your future together not your past. Work through the problems and you will build something truly amazing.

Of course you could also let your heart rule your head and confront her with the possibility of wounding her if she isn't cheating and blowing away your relationship if she is.