YSK that if your pet develops a limp or reluctance to use one of its legs that gets worse or does not improve over time, and there is no obvious cause, go to the vet sooner than later because it may be cancer. Why YSK: a lot of critters develop limps because of an injured tendon, sprain or problem with their paws. But if your pet starts limping without any reason you can identify, go to the vet for x-rays sooner than later. Osteosarcoma (bone cancer) is more common than people recognize and causes significant pain. If it is in the extremities, that can cause a limp or refusal of your pet to use that leg at all, and lead to muscle mass. Unfortunately it is fast spreading. If you can identify it before it spreads, amputation may save your critter's life and eliminate the pain. Don't wait to get it checked out if your buddy is not improving. Life on 3 legs beats the heck out of the alternative.