You seem to have erythema ab igne
It occurs after long contact with warm pad.
There is an abnormal changes in skin due to heat.
There is reticulated hypermelanosis
after erythema following chronic exposure to heat.
There is no perfect treatment.
Avoid further contact with heat pack.
Photothermolysis using different lasers may improve the appearance.
Some time it may disappear gradually without treatment.
I hope you got my answer.
Dr. Ilyas Patel MD