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If this has all occurred since the grooming, it is very likely that something happened then that set this off. Perhaps the tail got tugged on and there may be an injury to one of the vertebra in the tail. Or he may have experienced a cut or even clipper burn that is irritating the skin. If it is that painful, I doubt he will let you look at it, so he will very likely need sedation in order for a thorough evaluation of it.
I would schedule an exam with your vet. They may even need to xray to look for signs of injury to the small vertebral bones. If not anything else, it sounds as if he needs some pain relief. I would not offer any of the over the counter pain relief medications as they are not safe for use in cats.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.