We have three of these little guys and one of ours has the same thing going on with her back. They don't have a heat lamp so I know it's not a burn. She's had it since we found her and sometimes it's bigger than normal but I can't seem to find anything on the web about it either. We live in a really small rural area and our only vet doesn't deal with reptiles so we are stuck with looking on the Internet for the answer. We have had the frogs for about a year now and she seems to still be doing fine. Both the females we have are very big eaters and seem to be quite plump so we've cut back on their food.