It's pretty easy to see why she would hide it even if she wasn't up to no good. Just look at posts like this.
The majority of guys seem to think like this, and it's stupid.
Its not stupid with the information the OP has posted so far.
1) Dude was feeling down so he calls a person that he used to fuck. He basically needed a confidence boost and just happened reach out to someone of the opposite sex that he used to fuck.
2) She still has feelings for him. If you think the other guy is oblivious to this then you're lying to yourself. The guy got her in small space and made a move. He knew all along that she was still into him.
3) She responded to him positively. So far it really sounds like she would fight him off and out of the car.
4) In her tearful truth telling she also lets it be known that she thinks about still being together with him. Basically means she thinks about fucking him. If I was the OP I would be asking myself how many times has she imagined that the other dude was the one fucking her when the OP was fucking her.
If it was me and I wasn't breaking up with her I would tell her that she can't see this guy again just to see what her response is.