Hello, Borislav.
The burning acid that you are feeling is from bile. Everyone produces bile. J pouchers suffer more burning from the bile because we do not have the big colon to hold / neutralize the bile acids and we go to the bathroom more than people who have a colon. The job now is find a way to make your stools more solid, and not so many times, so it does not burn so much. Food choices will help, and so will Immodium.
Are you able to see or talk to your family doctor about pouchitis? Pouchitis (inflamed pouch) causes frequency, leaking, loose stools, and all this results in burning skin. Your family doctor can diagnose if you have pouchitis. If you do have pouchitis, you can get a prescription for Cipro, which is an antibiotic. It might be called something else in your country. For most people, Cipro works very fast, sometimes by the 3rd pill. I don't know if your surgeon is your only doctor, or if you also have a family doctor, but If you can't wait until you see your surgeon at the endoscopy, your own family doctor can diagnose if you have pouchitis and can write a prescription for you.