My surgeon explained to me like this...
In order to have plenty of room to operate within your abdomen, they inflate you with CO2 "to expand the area" he said. I think the dissipation of those gasses was the most painfull part of recovery! Oh, and when it a bubble makes it into your shoulders it's as bad as broken ribs! Ask your doctor, but I am sure this is what your experiencing. I was banded on 5/7 and today, the 12th, I feel that it may be just about gone, whoohoo!
Oh, the reason you don't burp is because it's not in your organs, it's just everywhere!