Certainly everything Steve said is paramount and a absolute consideration. Also, the substrate that you are using needs to be sifted and cleaned up right away. Try to spot clean on a daily basis with garter snakes bc they defecate and urinate quite often and pass urates as well. A little extra spot cleaning goes a long way in helping to avoid scale irritations, scale rot, and the like. A professional vet really needs to examine this to make the proper diagnosis. In the meantime, deep clean the enclosure and put the garter on solid white paper towels until you get a handle on it from the vet.