the humididity should be at 55-60% Constantly. Up it to about 70% for shed times, but generally, you do no have to, as 55-60% is fine for most normal shedding ball pythons. The low humidity is why you were having a bad shed, or may have had bad sheds previously as well.
What are you using to heat the Tub? I would suggest just cleaning the normal enclosure TONIGHT.
Is the Under tank heater on a thermostat? if not, you NEED to have one, or a dimmer. They cause burns. ( not always, but in this case, im going to assume yes, that is likely the case, which means you have to either do what kelly is telling you and get the soloution, or get some Neosporin ( WITHOUT the painkiller) and Clean the area Gently and apply lightly ( i have personally used Neosporin on a burn before, and the animal healed fine, but, it was a LIGHT burn, and not nearly as severe as this).
Call a vet in the AM, call a few if you have to, but make an appointment happen. This is probably going to be a little pricy if you have never brought the animal in before, but you WILL lose the animal if you dont know how to care for such a bad case.
You need to put some space between the bottom of the tank where the UTH is and the animal. Some newspaper ( take a few sheets and fold them up over the area inside the tank) and then put substrate down. You need to keep the snake from continuing to get burned.