I have the worst fluid retention (including before I was diagnosed with POtS). It’s the worst and usually is only resolved with the worst watery diarrhea in the history of time. I think my allergies also might have something to do with it as a trigger and being on antihistamines and asthma medicine. Have you been screened or tested for Cushing’s disease or liver/kidney issues? Also try looking up adrenal fatigue. It can lead to water retention