I am a fairly active, healthy 35 year old woman. I have noticed the left side of the back of my skull where it kindof sticks out on both sides seems harder and bigger than the right side. After a jog the other day, I noticed just below that about an inch on my neck, a small hard knot, under the skin, not noticable on teh exterior, not painful, does not move around, but it is definitely large enough to feel and find without much feeling around. I am concerned. What are some things this might be? Do you think it is imperative to see the doctor, or something that I need to watch?