I have been in a few car accidents and my atlas (the atlas is the top vertebra that your head rests on, your arteries ect. actually pass through your atlas and it can get rotated which puts strain on your arteries nerves that pass through it) goes out of place, gets rotated.    I see an atlas/orthoganal specialist to get it put back in place.  When I was doing physical therapy for my fibromyalgia there was one exercise that if I went over a certain weight it would shift my atlas and I would get an instant headache.  Would have to see the chiropractor to get my atlas fixed.    They were different than my typical migraine.  They always started on the left side of my head.  These atlas headaches almost always started at the base of my head, but they acted like migraines.