Have you been to a doctor? We can't diagnose you on a forum, or anywhere else since we're not doctors. I hope you are seeing a real doctor!
Postnasal drip could be allergy. Burping and refluxing and pain are symptoms, not a diagnosis.
I used to have a lot of burping after eating but my allergist had me keep a food journal and I found out it was food intolerances - not "true" allergies, but a problem nonetheless. Cider vinegar was one of my allergens, so I won't recommend that! I routinely recommend the food journal/diary because it can help you and your doctor diagnose if you have any intolerances. Look up the many posts on this by putting keywords into the search box at the top of the page.