I use a heater on my back, and also those wheat packs, and have done the same! Skin is all discoloured etc, and I was told that it wouldnt go back. It does fade a bit in time tho, as I discovered a few years ago when I managed to cope without heat. Back on it now tho!!
Tbh, I dont really mind, anything to help with pain, I get that needy! Another doc said they called it old ladies disease, he meant the old ladies who sit too close to the fire end up with mottled burned skin on their legs.