I had recurring severe pain in my ears as a child and was diagnosed with ear infections and given antibiotics many times. Suspect this wrecked my gut microbiome.There was no lab work done to establish the diagnosis I think. So it could have been just inflammation.My immune system throughout my life has been very effective wrt fighting off common infections like the common cold and the flu.
Did you take antibiotics since you first felt POIS ? I'm curious.