I'm not a expert... but it looks more like scale rot then a burn. You should a vet ASAP.

Also, you mentioned a heat pad? Was it hooked up into a thermostat? If not you could have cooked your spider from the outside......... saying he had the same pad she did which I AM ASSuming.

I'm sorry about your loss... all I Can give you though is my opinion and say to see a vet