Hi,
How are you? Are there other symptoms present? Can you elaborate on the character of the headaches? It is good that the temperature readings are normal. If there is no history of trauma to the head or neck, this could be due to neurological issues including primary headaches such as tension or migraine headaches. Have this checked by your doctor for proper diagnosis. Your complete medical history, direct physical and neurological examiantion, and diagnostic tests such as imaging studies may be done to determine the underlying cause. Take care and do keep us posted.
I would see a neurologist if I were you. If I remember right, headaches in the back of the neck are more unusual than other types of headaches. The heat issue sounds like a nerve that's being irritated. You might have a disk problem in your neck or something going on neurologically that's messing with your nerves or temperature sensors.
My son was diagnosed with neck-tongue syndrome which causes him to have a dripping water sensation in the back of his neck when it occurs. His is brought on by turning his head too quickly to the right or left which causes what he describes as an "explosion" in the back of his head that lasts about 30 seconds. It feels like a severe headache. It also causes a pulling sensation on his tongue so that he feels like it's being pulled backwards. They don't know exactly what causes it except that there is the possibility of some type of nerve involvement or entrapment during the episodes.