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Can you attach the photos here? How tall are you? How much do you weigh? Any weight change? Does anything make it better or worse? Is it getting worse? Are you aware of anything which might have triggered it? Any medical problems? Any surgeries? Any other medications including over-the-counter pills and herbs? Any family medical history?
Is it hard or soft?
There are three possibilities. 1. The skull can grow a hard bony lump (osteoma) from it's surface. This could grow gradually over a number of years, but would not usually be mobile under the skin. It is not likely. 2. Recurrent or severe sinus infections can be the cause of a lump or swelling. It is less likely because you don't have fever. 3. The fatty tissue under the surface of the skin can sometimes organise itself into a benign growth or lipoma, but this is not hard, although it is often mobile. It is less likely because you have pain. 4. It is a cyst of one your sweat glands in the skin (a sebaceous cyst). This would feel firm and mobile and would be painless unless it became inflamed or infected. If it is infected or infected, it would cause pain. It is more likely.
You should see your doctor or a dermatologist to have exam and CT scan.