My Toy Poodle has a recurring problem often after being groomed, and seems to be in discomfort. When he walks he is twisting his body and often stopping.
We've taken him to the vet previously on a few occasions and have to buy the pain killer for him and he is usually better within a couple of days. We have never found out the reason for the problem though.

This only happens after grooming?
Have you spoken with his groomer about this problem?
If so, what did they say?
Would it be possible for you to remain on site and observe him while he is at the groomers?
My first thought would be that he is protesting being in a kennel at the shop and pawing, jumping, digging.
After that, that he may be difficult for the groomer to handle or some of the positions are uncomfortable for him and he is fighting every step on the way.
If he is an older dog or arthritic it wouldn't take a lot to aggravate the condition.
Ask your groomer about staying with him, out of his sight so you can see how he behaves while there. If that is denied you, you may want to seek out another groomer.

I'd try another groomer once or twice, to see if it has to do with painful clipping.
And I agree, stay to watch.

If the dog is spinning around in pain after the groomers.. why not go to the vet for an x-ray of the back/legs? Clipper burn, I wouldn't think, would cause a dog to spin around in pain...