hello, i know exactly how you feel!! i have had sort of the same thing going on in my head!!!
you need to think about it this way,,, no matter what he did or didnt do, you were split up, he hasnt cheated, and i know it niggles away at you and its soooooooooo annoying!
maybe ask her some how??? do you know her at all? or better still someone that knows her??
or just ask him, have a heart to heart and tell him you still love him no matter what he tells you, you understand you were split up and you arent going to shout or be angry and maybe he will confide in you , may even bring you closer!
xx