"It's the sixth room on the seventh floor," Stefan answers, the smug sort of triumphant look on his face almost diminished by the sudden wave of guilt that washes over him, reminding him of what he was up to this morning.

He knows that husky tone of voice, and why Katherine wants to know where his room is. It's his own doing, for openly baiting her like he just did with pulling on his shirt in such a provacative manner. There's a part of him - a larger part of him than he wants to admit - that feels genuinely ashamed for having slept with Elena this morning. He'd cheated on Katherine with her doppelgänger, his ex. At the time, he hadn't been thinking of Katherine, but now... Stefan actually feels bad about it, regrets it, and finds himself wishing he'd controlled himself more when he met with Elena this morning, so that the guilt over what he'd done wouldn't be eating at him.

Stefan was used to feeling guilty about what he was doing with Katherine in regards to how it was hurting Elena. Never did he think he would feel guilty about doing something with Elena, because it was dishonoring Katherine. She couldn't know he'd done that; it was a very good thing he'd showered almost immediately after to rid himself of the distinct smell of both Elena and sex.