Remember telephone when you were a kid? One person started something told someone else and by the time it came back around it was just WRONG. Maybe you should give your gf the benefit of the doubt and assume someone got their wires crossed!
Your could confront her with what you heard but that could be like opening pandora's box and could sew seeds of discontent.
Ultimately if she IS plotting behind your back to get back with her ex - she's not worth holding on to.
I think the mistake a lot of us make is thinking the state-appointed shrink is our friend. - Jack Handy