No, it wasn't the pain. I believe he was distressed at seeing the other people in the infirmary with him. I've noticed that he has a terrible fear of Mr Lupin, and I suspect that that is the trouble - but he was not able to express his fear in any way at all. He kept insisting that he was fine, fine, utterly fine, and that he would just nip off to his duties - although it was excruciatingly clear that he was nowhere near fine.