It's always possible he has another hairball or 2 and that this is making him uncomfortable and so doesn't want to eat, although it shouldn't be stopping him from going to the toilet. It is also possible that he has something stuck down the back of his throat such as a blade of grass and he is being sick to try and get rid of it.

Either way, I would get him checked over by the vet to make sure it is nothing more sinister.