The fact that you are not getting the right cardiac output for your posture/activity, suggests possible autonomic dyfunction of some sort.
You say you keep a gallon of water by your bed, do you normally drink a lot ? because the effect of over-hydrating, if you’re not retaining the fluid is that you pass, in your urine, a lot of essential minerals. A blood test is not very accurate at reflecting the cellular levels of minerals. For instance, blood (serum) Magnesium can say you are ok, when at cellular level you are very depleted.
You need Magnesium to help produce energy. The gut needs energy, without it motility is slow. Often people lack B Vitamins when there energy is low too.
Have you had these symptoms for long ? Did they come on suddenly ? If so, do you know what triggered it ? Or have they come on slowly over time ? Does anyone else in your family get similar symptoms ?
Barbara
(UK)