I've come to a conclusion that it's most likely a burn because I moved her tank.
as I said before, I previously had her tank on top of a shelf with planks to give circulation underneath the tank to regulate the temperature, but at my dad's suggestion moved her temporarily to the floor.
Obviously the heat could not escape and it got too hot for her when she burrowed under her substrate, and since I did not handle her for two days, I did not see it