Damon hadn't meant to spill the beans about being a vampire to Elena. Really it wasn't his fault. His brother should have told her what he was so in reality, this was all Stefan's fault.
Damon was ecstatic that Elena had agreed to meet him so he could show her that he was indeed a vampire. He didn't blame Elena for not believing him. To most humans, vampires were just the product of scary books and movies.
But just as he had Elena right where he wanted her, Caroline interrupted his fun. It was no matter, he had taken care of the Caroline problem. She wouldn't be interrupting his fun again. After he finished, he made his way to Elena's door and knocked. When she opened the door he gave his best grin. "Who? Oh! Let me guess. Caroline? She's......busy." Damon stepped over the threshold and into her room. It looked similar to his own but he did a quick look around to make sure there was nothing hiding in the shadows.