It's my main symptom other than that I'm ok apart from my muscles do twitch when I'm relaxing 
, I basically get a sensation in the back bottom of my head, like its tilting/falling backwards, it feel like it spreads to my neck and top of back, it comes and goes once a day and lasts a few hours, is it all just anxiety, or something worse?
I would say tiredness. In my case the head tilts then I am asleep in my chair.
How is your blood pressure?
That's ok
Have you ever considered going for a real massage? Amazingly it is very helpful.
you are describing tension. Massaging the area will really feel good.
Can tension give that falling sensation?
Tension in the muscles, im sure normal tension too, but thats not what i mean. Tight muscle tension. whether posture, or sleep position..Physical tension. when muscles get irrated, inflammed or very tight im sure they can create many strange sensations. That might be worth just going to a doctor and discussing it. Im not sure which specialist to see though.