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).
I found a bunch of threads regarding sore scalp & hair growth. Apparently it's pretty common to have sore scalp when you're having a growth spurt, or about the time you normally are due for a touch-up (for relaxed ladies). Personally, my crown area gets tender at about 5 weeks post (where I am now). That part of my hair also has the most new growth. Nov 6, 2006.