Greg was worried with Pansy. He hadn't spent much time with her at the wedding (in fact, outside of saying hello to people, he hadn't interacted much with anyone besides his sister) so he had no chance of asking her what was wrong. And then, she locked herself up and didn't talk to anyone. Since his mother had been spending most of her time in her own room ever since the Battle, even refusing to join them to the wedding, Greg was understandably worried. At least his mother allowed herself to join them at meals, which hadn't been Pansy's case.
In spite of his worries, he didn't feel like he should intrude. What if trying to coax her into eating made her mad at him? Either that, or it would make her feel worse. Either option was unacceptable.
Finally, though, she responded to his knocking. Timidly, he entered her room, a tray of sandwiches in his hand.