Help! what's wrong with my pet rat???

He could have just hurt himself? Or maybe, just a possibility, he may have had an allergic reaction to something. Have you recently done something with their cage (disinfectants, soaps) or given him something different in his diet? Or have you cleaned the room he's in with some kind of spray, that could have gotten into the cage.. Just to be safe, i'd take him to the vet. They'll be able to find out whats wrong with him. Best of luck ♥