We had a similar issue with our Saint Bernard, except the tumor was closer to the joint and the swelling (hard mass) put pressure in the joint causing lameness. I could move the foot, press on the bump and no reaction, trying to walk on it caused pain. It appeared all of a sudden - I had hoped for a stress fracture, thinking that was the worst case scenario. Our vet took one look at the x-ray and said it was sarcoma - could tell by the texure of the density of the mass compared to surrounding bone, and the shape (thickness) of the membrane. I hope in your case it is better news -
