He would like to blame the pensiveness on a leftover hangover, but that wasn't really the case. He hadn't drunk enough for it to last longer than halfway through transfiguration that morning. It likely had more to do with his discussion with Leon on the journals that weekend.
He hadn't handled it as well as he could have.
"The preference," he said, "would be for no one to take the fall. She shouldn't be in there at this hour, but we do need to make sure she's far away."