First of all, you shouldnt believe everything you hear...
Secondly, i agree with Kid, you really do need to talk to her...maybe even re-evaluate things because from what you have said in your post, it sounds like you dont trust her
...i understand its easy for someone to plant a seed of doubt in your mind and it can grow very quickly...but it might be an idea to think things through...what are you both getting from the relationship? What do you want? Do you still have feelings for her? etc
Anyway, I hope things work out alright!