I have heard that the reason why animals don't get painkillers after some surgeries is so they won't overdo it and hurt themselves. It sounds cruel, but it makes sense. An animal that is hurting a little will tend to take it easy and rest, if there is anything masking the pain, they might act normal and do further damage to themselves.

My male kitties have sailed through the operation and return to normal within a day or two. It barely slows them down at all.