[It's a heavy sort of sound. After all, that "oh" has a lot of weight to carry around. She trusted him and he lied to her quickly and seemingly easily and even though she though it sounded strange it never occurred to her not to believe him. It didn't even occur to her how much she trusted him until he made it clear he'd abused that trust.
On the other hand he's the one bringing it up to her—trying to make it right. He didn't have to do that. She probably wouldn't have found out differently.
She pushes herself back a little further, dropping her hand from his arm into her lap. It's not a sudden movement and she doesn't push him away, but it's clear she wants a little space as she digests that.]