For a moment, Sans forgets why he's being singled out here. But only for a moment. It's not like he doesn't know who Lapis is by this point - he's been in the apartment, and he's seen her around. She is nice to his brother, which he has found is an instant way to gain his favour recently - it's a strange feeling to have, in regards to a person you didn't have any memory of a couple weeks ago.

He makes an exaggerated gesture at himself, clearly asking 'ME?' as if he is somehow unsure that he is indeed the one Lapis is looking at. He's not sure what he's supposed to do about it either, but he's interested in seeing where this goes.