What I gathered from the sequence of events was that Rinny's change was not just a physical one but a mental one. Asking her about it reveals that the change brought a rush of new sensations and feelings and she simply got lost in the moment, like a sudden high from which she couldn't quite come down. Once you talk to her later you notice she's sealed away that power because she sees how dangerous it makes her and doesn't want to go out of control again. It's almost like she's got a bruce banner/hulk dichotomy going on, except it's lust instead of anger.