One of the main causes of sinus drainage is simply being alive. Every day the human body produces several liters of sinus fluid and mucus, and all of that natural liquid must go somewhere.
It is a area of tenderness of the scalp in which people also sometimes feel stabbing headaches. Others just feel very sore and tender to the touch. It is felt to be caused by irritation of the connective tissue which attaches the muscles of the scalp to the skull (cranium).
My scalp (especially the two sides of my crown) feels very tender, itchy and painful. I have ignored it and hoped it would go away, I thought maybe it was a phase, but it has been going on for too long now. I had a baby four years ago, so it cannot be that.