Some owners report an unusual form of lameness in their dogs. Characteristically, they will say their dog was suddenly very lame in one back leg, dragging the leg behind them with the leg stiff and straight. They will then say with wonder that suddenly the dog became normal again. Often they ask if the dog had a fit.

While fitting does sometimes causes this problem, dislocating kneecaps (luxating patellas) are a much more common reason. This can occur in any breed. It is caused by the sideways movement of the kneecap. When the kneecap moves out of position, it acts like a wedge and tightens the ligaments around the knee so that the animal cannot bend its leg. Suddenly the kneecap slides back into position and the dog can walk normally again.

Maybe this will help