What should I do if I have had red patches without pain for months after prolonged exposure to an electric heating pad used for a stomach ache?

Hello, it is possible that the lesions you describe are associated with erythema ab igne, a discoloration of the skin in response to repeated or prolonged contact with a heat source. This usually does not require medical attention, unless there is pain associated with it. Unfortunately, there is not much you can do except wait for the lesions to fade. It is important to limit exposure of the skin to direct heat. If the skin does not return to normal in a few months, or if you are very uncomfortable with its appearance, please consult a doctor.