You have, apparently, some variant of POTS (positional orthostatic tachycardia/arrhythmia) and it might be due to impaction on the Vagus nerve or loss of tone of the Vagus nerve during certain postures. It could also mean you need an MRI to see if your spine is okay. I'm only guessing.

I would urge you to get some advice, and an assessment, from a postural specialist physical therapist. You may need a 'fascia release' technique, ligamental release or realignment, muscle tone improvement....maybe all of those, but I think you should really do this to find out if you can get even three or four degrees of correction. It might make all the difference in the world.

The other possibility is visceral adiposity....internal body fat deposited around and in between your internal organs. In certain positions, this can come to bear and cause problems on surfaces, blood vessels, nerves....