GI symptoms (i.e., nausea, vomiting, abdominal pain, constipation, diarrhea, etc.) are common in POTS patients, but the exact reasoning isn't well understood. However, it's believed that since POTS is a disorder of the autonomic nervous system, sympathetic autonomic denervation can occur and result in neurotransmitter imbalances. (Such as not enough norepinephrine in one place, or too much in another, ultimately causing problems.) Also, many POTS patients have comorbid conditions, which also result in further symptoms.

I have gastroparesis and slow transit bowel. Not sure if it's POTS related but I know it's EDS related. I think gastroparesis can be caused by POTS. It can also give functional stomach issues like IBS of both types. Functional doesn't mean not real just no obvious cause I believe.