It looks like you have developed skin damage (erythema ab igne) due to repeated heat exposure from the heating pad, causing dark patches, peeling, and rash.
Next Steps
Stop using the heating pad immediately.
Apply a soothing moisturizer (like aloe vera or Cetaphil lotion) twice daily.
If inflamed, you may use hydrocortisone 1% cream for up to 5–7 days.
Protect the area from sunlight to prevent worsening pigmentation.
Health Tips
If the rash worsens or pigmentation doesn't improve in 6–8 weeks, consult a dermatologist for further treatment.