If she'd been assaulted before there are any number of things that could be going through her head. I'm tempted to say she's afraid of how she reacted, may now be having thoughts that she is "sick" (could be an after effect of the assault, making her think she deserved it etc for having the desires she does now). And having these thoughts could be leading her to "selective recall" regarding that night. She may be ignoring how much she enjoyed herself that evening because of the fear that enjoyment later gave her. The fact that it happened several hours later is probably due to the fact that she was coming down from her emotional high and started to rationalize what had happened.

Now I'm not saying any of this is true, but I'm fairly good at reading people/situations and this just seems to be what happened. There really isn't much you can do in this situation, because none of it was really you. She needed to know herself better and make the two of you more aware of the, even potential, fears she had.

Anyway, hope that helped a bit!