Sherlock blinks at the look on John's face because it's not at all what he was expecting. It just doesn't add up with everything else, all the things he'd noticed... or thought he'd noticed. John hasn't even said anything yet - has barely tried to stand - and Sherlock is replaying it all through his mind to find where he'd gone wrong. He knows that here, his margin for error is significantly higher than with other areas - he's not good with people when it comes to things like this, but he'd thought this was different.

So when John is trying again to stand, Sherlock lets him go and lingers on the floor while he stands, watching him climb to his feet. He's only half-present in watching him because he's mostly got his focus turned inwards, so he looks a little vacant and perhaps a touch dismissive, the way he tends to when he retreats into his mind palace. He should say something to John, maybe. It's an almost awkwardly long pause before he manages to say a very belated, "Of course."

Then he's standing and adjusting his clothes as though he'd just stood from being seated in a chair, and makes his way to the sofa, where he sits down, apparently going right from nearly humping John to casually relaxing without missing a beat, like nothing strange had happened at all.