CWB

a sticky situation at best . the way i see it , you are damned if you do and damned if you don't ...

if you admit to what you did , she could claim that you had no right to do as you did , get bent , and walk .
she could continue to lie to you (as she obviously has been) and come up with some kind of story to cover her arse ... "i was just playing with him , you're the one i care about" ... ad nauseum infinitum .
you mentioned being "suspicious" ... is this based on other indications or is this stand alone in it's nature ? i certainly would be hard pressed to not be suspicious given that she "broke up" with him long ago and still continues treat him in an all to "personal" manner .

my question would be ; can/could i trust her in light of what i know she has done ?
it seems to me that she lacks a sense of honor and has no/very little respect for you in the areas that weigh the heaviest .
in a way it might be a good thing that you found out now as opposed to several months down the road ... less pain/hassle this way .

good luck on whatever you choose to do .