l had a panther that had the same thing. Turned out that it was caused by an infection. The bump was basically hardened puss that had attached to his ankle. I didn't even notice it until one day he wasn't using that leg like normal. I had to inject Ceftiofur subcutaneously (just under the skin) into his back legs over the course of a week. The infection greatly decreased in size during that week and he now uses his leg like normal again. I am not saying that this is the case, but a possibility. You may want to consult a vet.