Your body is reacting to the toxins and bacteria from undiagested foods remaining in the stomach for extended time.
You must have developed nutritional deficiencies which should be checked ASAP!.
Forget the drugs for the hives and concentrate your efforts eliminating the cause, the only approach which will bring you effective resolution.
Conditions/imbalances preceding your diagnoses must be investigated and treated, as well as those developed after your diagnoses.
Are you diabetic? If yes, insulin can cause intestinal damage , so you should take it after meals!
Have you ruled out hypothyroidism?
This could be one of the underlying causes in Lupus, Gastroparesis and possibly a factor in POTS (other factors are diabetes, hypovolemia, adrenal fatigue and others).
Other things to specifically look into is magnesium deficiency (tissue levels), associated with vagus nerve damage, which is a likely causative factor in Gastroparesis. and other deficiencies like vitamin D3, digestive enzymes and methylocobalamin B12
Probiotics of the highest quality would also help, specially if your intestinal flora is compromised .
Best wishes,
Niko