I would be pleased to answer your question .
You seem to have got Irritant Contact Dermatitis
or Pigment Contact Dermatitis.
It is not due to the heat but due to irritation of the skin due to some component your skin is not used to.
You will need to apply a mild steroid
ointment which you can get by prescription from your GP or Dermatologist
It will take 15 days to 20 days for the lesion to resolve.
Thanks and Take care.