If I was dating a girl, and someone told her I cheated on her (even if not true), and she didn't go absolutely ape-shit nuts about it instantly and try to kill me or herself, then I would kind of get the impression that she wasn't that interested in me .... does that make sense?
If she believed the roommate who said you cheated on her, then she'd be real mad, yes? And yet, she wasn't, she instead snuck around behind your back, yes? Which means she either a) doesn't care that much about your relationship or b) your roommate never said that and she is using it as an excuse or c) he said it but she doesn't believe it and she trusts you, but if so, why is she bringing it up?
Although really I think she is just looking for attention, some girls want more attention, from lots of people, not anything sinister, she probably enjoys having the roommate listen to her whining, he is a SNAG perhaps (Sensitive New Age Guy), trying to give her emotional support but she just likes that for attention but is actually dating a real man, not a SNAG.
Thing is though you are a bit sensitive, maybe you need to just go "well, stuff you babey" and then she will probably like you more, when you dump her. Because you'd be strong, which is probably what she really wants. ... because that would show that your exclusive relationship is so important, it is all-or-nothing, and she better shape up or ship out.
You've been together 2 years it is quite a while so she must like you somewhat.