After cleaning my American Bullfrog tadpole tank, I noticed Poe has an odd swelling behind his relatively new back leg. It's causing him to list to the right when he is resting. I've included a couple of pics (the swelling is located just behind his leg, at the base of his tail). I am really hoping that it means nothing and maybe he just has to have a major bowel movement....? If anyone has experience with this, I would greatly appreciate any advice!