I have a 9 year old spayed female Rottweiler who is overweight. Two weeks ago my dad took her for a walk to the grass field at night. At the field, she chased and played around with a soccer ball. After playing, she limped back to my dad, with her back right leg not touching the ground. My dad did not see what happened, since it was dark. Two weeks later, she still limps with her right back leg still raised up. What is wrong and what can I do?