I’ve been through this with a bunch of experts and hoping I can shed some light on why this happens.
Health anxiety is more than a thought process, it manifests itself at a neurological level (subconscious).
I had the jerking awake just as falling asleep, waking up in middle of the night panic, and morning terror as I woke up. I also had it just out and about walking.
What i realised about the walking was as I was scared of exercise and palpitation episodes. when my foot hit the ground in such a way as you get some foot falls rattle right through your body, my nervous system interpreted it as a hard palpitation heartbeat, and the automatic anxiety response was generated. all this happens Ahead of your rational thinking, as the pathways to the autonomous nervous system are quicker to fire.
it was also explained to me that as you fall asleep, move between sleep phases or waking up physiological changes happen to breathing, your heart rate, and things like chemicals to keep your muscles still while asleep.
with heightened underlying anxiety all these can be misinterpreted as a fight or flight incident, and the resulting panic feeling ensues.
this went a long way to helping me understand what I was experiencing and putting in context.
I hope this can be some help to others. take care.