Stumbled onto this forum through a google search, but wanted to offer some relevant info: look into SIBO (Small Intestine Bacterial Overgrowth). A friend of mine found out about it due to her B12 deficiency and ended up telling me to look into it as well (after finding studies, etc showing increased prevalence in POTS patients). It can cause a lot of GI symptoms that overlap with POTS, but uniquely causes malabsorption, vitamin deficiencies (esp iron, B12, and/or K), and weight loss/gain.