POTS is a form of neuropathy. It is part of the spectrum of autonomic neuropathy.
HOW you got this is my question. Since you did not have this before the bypass surgery, I am wondering if something occurred during or shortly after that surgery.
Midodrine or Fludrocortisone along with compressive stockings are what they use to control the condition.
Again, POTS=Dysautonomia=a form of Neuropathy. Your case seems particularly severe to be classified as POTS, since most are not bedridden with this kind of severe pooling, but, by all means, question, question, question.
Hugs to you. You hang in!