Hi, I Am 61 Years Old, 6'3", 169 Lbs. I

Hi, I am 61 years old, 6'3", 169 lbs. I fell asleep on my back with a heating pad (older model, no auto shut off)
on low underneath my lower back for 3 hours. I just woke up and amy skin is pink but not blistering. However I have a cough I did not have. Could I have harmed my lungs? I did have two layers of clothes between me and the pad.