I used to get really severe fatigue from malabsorption caused by too many years of undiagnosed coeliac disease. That also had the effect paradoxically of causing a leaky gut syndrome at the same time. The explanation I got for that was that in a leaky gut protein molecules don't get totally broken down to the smallest amino acids before they leak through into the blood stream, and some protein components have a real sedative effect on the brain (similar to morphine type substances). That really explained the brain fog for me - where I couldn't stay awake after a meal with protein. I do get a bit fatigued now, mostly in a flare, and I wonder if some of that is pain related. Nowhere near as bad as when my gut was really destroyed by the coeliac disease (it got to severe villous atrophy stage on biopsy).

I have often wondered if I still have some element of orthostatic hypotension - not so much in blood pressure dropping, but heart rate increasing.