Any kitty cat lovers know what this might be?
He might not be feeling well - just like when people get a vaccine and can feel a bit tired and sick afterwards. If a cat's not feeling well it can act grumpy or 'aggressive' just like some sick people don't like being bothered and just want to lie comfortably somewhere until their sick feeling goes away.
If it persists or it seems to be a very big change in personality, take your kitty back to the vet or give them a ring and just check the symptoms are normal. (Also, keep an eye on the litterbox- that he's using it normally/regularly)