I believe that you are suffering from occipital neuralgia, brought on either by infection from your sinuses or from pressure by the lump on the back of the head. It is likely that this lump is a sebaceous cyst - a harmless cyst from a sebaceous gland but possibly lying just over a nerve called the occipital nerve.
The occipital nerve supplies the muscles of the back of the head and neck and also sends fibres to the side of the head reaching the skin and muscle above the ear, to the sinuses and near the eyes.
A headache with tension of the muscles of the back of the head and neck can also cause pain over the point from which the nerve emerges from the skull.
Here is a very good image of the occipital nerve emerging from the skull:
- http://gulfcoastpain.com/onb_Info.htm scroll down to the image midway on the page.
The pain goes away when you massage the area because the muscles relax, releasing pressure over the nerve.
Any pressure on the nerve, either by tense muscles or by a compression at the level where the nerve emerges from the spine or or by changes in the blood supply can cause pain and headache.
The standard treatment is injection of a steroid and local anesthetic at the base of the nerve. Possibly in your case, removing the sebaceous cyst will be necessary.
I hope this helps.
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