I have been having an headache at the base of my head and behind my ear.

You must know that headaches are practically infinite in their variety!
Countless go unexplained.
We can take a few guesses:
Ear infection
Tooth infection
Poor posture, usually sitting at a desk peering at a computer
Sleeping 'wrong' or sleeping in AC, etc
Heavy purse, bag, carrying child on hip around the house
... and as always, something rare that you would investigate after all the usual treatments

Try ibuprofin (inflammation) and Tylenol (pain) together or alternating, a warm tub or shower soak, a heating pad with a moist washcloth, and report back tomorrow, or see a doctor. And a dentist if you haven't lately.
If someone can massage your neck, often they can feel tight muscles.