Glad to hear that you got her to a vet and she is on her way to recovery. It definitely looked like a burn to me...and I know that my girl loves to shove all of the bedding out of the way and lay right on the glass.
You definitely need a thermostat. But after an expensive vet trip, at least start with a rheostat. A good thermostat can be kinda expensive but a rheostat can help tie you over until you are able to get one. But if you end up going with a rheostat, be aware that even on the lowest setting my heat pad would get up to 100 F, and I'd have to turn it off sometimes to keep the temps where they should be. They are definitely not a long term solution, but it will keep the temps lower.
Hopefully she will heal quickly and you can get the heat sources set up correctly so you won't ever have to see her poor belly like that again!!
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