There is such a thing as excercise induced cardiac collapse, mimics epilepsy, not a Brittany issue to date. Any injury history with that joint? Sounds more like a recurrent issue from an old injury, that the dog has never let heal completely. Seems like if it were an arthritis it would be manifest in more than one joint. Recovery seems fast and complete. Try managing with anti inflamatory, ( banamine, aspirin), This from a old amatuer practitioner on farm horses and dogs. I'm no vet, and the professional knows more than I do, however, I like to eliminate the less exotic and dramatic issues first and second vet opinions are good. As I reread your post and note the age of the dog, check to see if his growth plates are closed in that joint, I wouldn't rule out a sibling bit him hard in that joint as a pup, I've seen it a lot, and it takes a long time to heal.