My dog chases his tail, growls, and cries out while pooping. Vet says he's fine- xrays & the works. Ideas?

He didn't initially have this behavior when I rescued him in June. He has had a an exam, xrays, and antiinflammatories. He has been on narcotics for a week and his behavior hasn't changed. Help! Its so sad to watch. He looks like he is in a dogfight while he is trying to have a bowl movement.