No, I had no real reason to believe he was injured by a bite. It was just the only change. They have plenty of substrate, but sometimes he digs to the bottom in their hiding spot and I have to pull him off and even out the substrate again. I figure that this is a sign that their tank is too cold, but I've been working on that. Is there perhaps a trick to keep them from trying to lay directly on the pad?

The main reason I didn't think it was a burn was because it's on his back, which he clearly doesn't lay on.