Lexie was clearly not herself as she stormed out of David's apartment, unable to explain her sudden, intense repulsion. The feelings died down a little as she left, not even bothering to say goodbye as the door snapped shut. She just had to get out out of there and away from that slimy bastard. Had she been thinking clearly, it might've occurred to her to be suspicious about the fact that at the beginning her feelings had been quite different at the beginning of this conversation than they were at the end, but Lexie wasn't thinking clearly so the thought didn't even enter her mind. She didn't know it, but she wouldn't be thinking clearly for a long while.
As she exited David's building, though, she couldn't help but think that there was something important she'd left behind. Stopping quickly, she did a little inventory of herself, making sure she had her purse, wallet, cell phone. It was all there. Weird. It was probably all in her head.