Do you have an under tank heater? How are you controlling it if you do use one. It could be a burn. What kind of substrate do you use? How often is it changed/inspected? A vet is always the best choice in a situation like this, but until then - take out your substrate and replace with paper towel or newspaper. Apply a triple-antibiotic ointment such as neosporin with NO PAIN KILLERS in it.