It can be very difficult to get a diagnosis for lots of things, especially those involving the spine. Medical science has a ways to go yet.
Of course, there may be multiple causes, interacting, just to make it interesting.
I had bad headaches for 20+ years, and the only treatment that worked for me was remedial massage. My back muscles were knotted and were pulling down my neck muscles, giving me relentless headaches. Started this treatment early last year.
Now I have found an even better one. Magnesium chloride. I drink about 1-2 teaspoonfuls of it in water every day, and the difference is amazing. I no longer need a massage every 2 or 3 weeks as my muscles are not knotting up any more. If you have a magnesium deficiency, estimated to affect 80% of people, then this treatment may well help.
It is cheap, easy and harmless.