Firstly, just a thought but, after having a caesarian abdominal operation, my digestive system stopped for 5 days (OOuucchh!) They kept coming in each day and listening to my tummy – no joy. Then a midwife in her senior years said, you need an abdominal binder – which strangely enough, stopped the pain and worked, yey! My daughter being born the day of the caesarian will be 30 next year, so this happened long before I knew I had POTS but, I’m just wondering, as an abdominal binder is one of the treatments for POTS, whether this might help.
Secondly, I know that nausea can also be a sign of brainstem compression, have you tried wearing a rigid philadelphia neck collar ? It might reduce the nausea, if it’s being caused either by a chiari malformation, or by cranio-cervical instability, both of which are relatively more common amongst us, than in the general population.