From My POV..... I would say that it is possible that she is carrying on with someone else... and not so much because she has cheated before, but due to some of the other reasons you mentioned... those reasons (if true) seem to be saying I am up to some thing that I do not want my b/f to find out about.

NOW - in all honesty you will never know until you talk to her about your concerns, or catch her read handed.

LoVe,
Rhapsody -