Just based upon what your state here, it seems like autonomic neuropathy is a very real possibility. The rushes you experience when standing or bending, then standing are typical with postural orthostatic hypotension. It would be very easy to confirm this by taking orthostatic BP's (laying, sitting, then standing). If there is a significant drop in systolic pressure when standing (greater then 20 mm/hg) then you obviously have some autonomic dysfunction. Your heart rate likely increases significantly as well in compensation for the drop in BP.

I would suggest you inquire about full autonomic testing (tilt table, SQART, 24 hour BP, holter monitor, to start with). UT Southwestern in Dallas I believe has these basic autonomic testing (as I was initially tested there many years ago).

As for the cause...it can be many things. I'll assume you've ruled out the basic tests for diabetes. I would therefore, start with autoimmune testing (full work up). This sounds more likely since it obviously was rapid onset. Do you have any other symptoms that many not seem significant (but are new) that might help narrow down an autoimmune disease? Anything odd...like joint pain, profound fatigue, dry mouth, eyes, rashes, swellings, redness, etc?

Have you had B12 checked?