ETA: Some reviewers thought it was a reference to Snape performing Legilimency, but of course it doesn't take a mind reader to notice a dreamy-eyed girl.

He tells her to stop staring at him, and doesn't look at her when she comes to see him after class. Perhaps this was because he no longer needs to read her mind, because he's decided to stop reading students' minds, because he was doing so at the headmaster's bidding and he's broken with Dumbledore.

Wildly conjunctive and supported by a hair's breadth of evidence, but I don't have a better guess. Nothing else seems likely to have a dramatic effect on Harry's story.