Oh, when Kimber was about Sassy's age she went through what was called "puppy pano." As far as I could tell, she would run and play, start limping, we'd have to go on rest for a week, then she'd do it again a few days after she was released from rest. Since she was in her highest energy stage, it was a bit of a nightmare. It did eventually stop happening. I hope Sassy heals quickly and forever-ly!