Couple of things about doing stuff after competing. I actually remember this issue coming up a while ago. It is risky to train or do higher level activities after competing, particularly with flyball. Doing no activity though isn't the solution - going for walks, and not running, is the best activity.
I say particularly with flyball because of the issue of adrenalin, which is pretty high at a flyball comp. One of the issues of not resting after a comp is you aren't allowing the dog's adrenalin level to drop naturally. Even though the dog may feel amped up and want to run, you shouldn't feed into that. Our dogs are not working dogs, if they were their levels would drop quicker. Although nothing is like the high level bursts of energy at a comp.
Then there's the issue of muscle fatique. This is something the specialist had told me with Tully. When the muscles are fatigued they are much more prone to small tears - particularly around the shoulder area. Agility and flyball both require the dogs to do unnatural twisting, so ball chasing is the absolutely, utterly worst thing you can do after a competition - because you are likely causing a lot of tearing around the medial shoulder area. In fact, ball chasing is a bad thing to do with any dog that trains in flyball or agility. And from the continuous lectures I would get from the dogs' physio - obsessively chasing balls (or Frisbees) - is not something you should do often because of the same risk with shoulder tears.
And it isn't because a dog is young - in fact as they get older their muscles become less elastic and more prone to injury. Although at least as they get older they (sometimes) are less silly about throwing their bodies around.
There was another reason you should only do walks after a comp, but the cleaner has arrived so will have to save it for later.
I must say, I think all performance dog owners should visit a physio for advice. Chiros may be vets but physios (well here anyway) are trained to deal with athletes so can give you a recovery routine for after comps which is really helpful.