Any idea what caused this?
Welcome to BYC. If you still have her body, you may want to open up the organs in the abdomen to look for white deposits that may be gout. It can cause the swelling of the feet. Mycoplasma synovitis can also cause swelling in the feet and leg joints. Sorry for your loss, and if you do a ncropsy, please let us know what you find. Here is a link about gout:
Edited by Eggcessive - 9/30/15 at 3:34pm