I have pain when i touch the backside of my head, above the base of skull, but behind ear. sinus infection/cavities may exist. advice?

Not sinus: No sinuses in that area. Probably a cervical neuralgia or muscle pain from the spine. see a neurologist

Unlikely sinus: Pain in the back of the head is unusual for a sinus infection.This would be more likely an ear infection but this is unlikely as well. This could be a mastoiditis which is rare but plausible. This can come from a middle ear infection not treated or inadequately treated. Other causes could be referred neck pain. See your doctor very soon. Best wishes.