Keep in mind that you can have dysautonomia without having POTS. In order for it to be POTS, your heart has to speed up drastically in response to the position change. That's the "T" for tachycardia in POTS. I think doctors generally say your pulse has to go over 100 to be tachycardia. When I first started passing out, my pulse never went over 100. Now, sometimes it does, sometimes it doesn't. Either way, I end up on the floor! It's still possible to have orthostatic intolerance, PVC, etc. without the tachycardia. And these symptoms still need to be addressed with medication, physical therapy, compression stockings, or other treatment.
While there may be an initial drop in blood pressure while standing in P.O.T.S., some people have the hyperadrenergic form, and the blood pressure can actually go high standing.
I believe this is the form of P.O.T.S. I have, though I have not been formally diagnosed with this kind, only diagnosed generally with P.O.T.S.. The Mayo clinic had an article stating that those with P.O.T.S. with higher levels of norepinephrine while standing had a "significantly more beneficial respnse to treatment with beta-blocker medications." However, I tried one of these and didn't fare so well, I don't why.
I did better with a calcium channel blocker. But here, the cardiologist didn't want to give me a prescription for it, thought it would make me worse. In recent weeks, I have been feeling significantly worse than I have in a long time.
Where were you evaluated for your POTS symptoms? POTS, and other forms of Dysautonomia can be very difficult to diagnose. If everything else has been ruled out, and you still feel strongly that your symptoms are autonomic; then you may want to look into an academic teaching hospital, or a Dysautonomia Specialist if you haven't already.
Diagnosing Dysautonomia:
http://www.medhelp.org/health_pages/Neurological-Disorders/Diagnosing-Dysautonomia/show/827?cid=196
Dysautonomia Specialists:
http://www.medhelp.org/health_pages/Neurological-Disorders/Dysautonomia-Specialists/show/717?cid=196
Most patients with Dysautonomia will actually experience lowered blood pressure upon standing, rather than laying down. Do you have a home blood pressure monitor? It may be helpful for you to make a log of your blood pressure readings.
You mentioned feeling as if you had a seizure... Have you had an EEG to check for seizure activity?