Hi. I'm sorry for the delay in response. I am recovering from a prolonged hospitalization myself and am hit/miss on here lately.
(See here: http://www.medhelp.org/posts/Dysautonomia-Autonomic-Dysfunction/A-long-overdue-and-massive-update/show/2019178
Now, as to your question about passing out lying down. Yes, I've had it happen on rare occasion. Mine has never been quite as dramatic as yours, though, where you are at rest for such a prolonged period of time in the supine position before passing out.
Have you had your Dr. appointment yet? If so, what did s/he say about it? They may want to do some tests on the vessels that circulate blood to your head to see if there could be an issue there. Apparently the issue may also be related to epinephrine/norepinephrine balances (catecholamines):
I found this tidbit in a journal abstract that you may want to show your doc:
"Patients with syncope while supine also had sympathoadrenal imbalance before loss of consciousness. Sympathoadrenal imbalance precedes tilt-evoked and spontaneous neurocardiogenic syncope and correlates with concurrent skeletal muscle vasodilation. Sympathoadrenal imbalance may contribute to hemodynamic derangements precipitating neurocardiogenic syncope."
The author, Goldstein, is one of the leading authors in the field of catecholamines (a key component of autonomic function), so this is likely a good article.
Here's more from the article, if you're interested:
"Syncope while supine
Sudden loss of consciousness occurred in 4 patients while being monitored in the supine position. At baseline, arterial plasma concentrations of norepinephrine (1.3 ± 0.2 nmol/L) and epinephrine (0.29 ± 0.09 nmol/L) were normal (1.3 ± 0.1 nmol/L in 77 normal volunteers and 0.34 ± 0.03 nmol/L in 45 normal volunteers). During spontaneous loss of consciousness, all 4 patients had an increase in plasma epinephrine levels (mean 3.8 times baseline), and no change or a decrease in plasma norepinephrine levels (mean 0.86 times baseline), so that the plasma epinephrine:norepinephrine ratio increased substantially (5.3 times baseline).
In 1 patient who had blood sampled repeatedly, as called for in the protocol, arterial plasma epinephrine levels began to increase and norepinephrine to decrease about 3 to 4 minutes before onset of mild hypotension (Figure 3). After about 8 minutes, blood pressure and plasma norepinephrine reached a nadir, plasma epinephrine peaked, and the patient became symptomatic. Subsequently, blood pressure and plasma norepinephrine increased spontaneously but did not reach baseline by 30 minutes, although by then plasma epinephrine had declined to baseline."
As for what you can do about it, much like syncope in general one can consider a combination of medication, diet and exercise modifications, and non-pharmaceutical treatments. What is your current regimen for your POTS and syncope?