I'm guessing he's just in a bad mood if he's never done this before. If you'd just had surgery, you'd be short-tempered, too. George sometimes acts like this (he doesn't bite or growl, but gets protective of his food), but other times I can take his food right out from under his nose and he just sighs. I'd say give him a day or two to feel better and if he's still doing it, THEN start doing some behavior modification things like Steinar suggested.