Where does the gas that we belch or burp come from? The gas brought back by belching (as opposed to intestinal gas, which can come from the digestion process) comes from swallowed air. We all swallow some air when eating and drinking. Most of the gas mixes with the stomach contents and either enters into the small intestine or is belched back. The air that enters the small intestine is either absorbed or it may continue through to the large intestine and is then passed rectally. Individuals may swallow more air (and thus increase stomach gas) if they have post-nasal drip, chew gum, have poorly fitting dentures, suck on hard candies or smoke tobacco. Drinking carbonated beverages or eating rapidly also can increase stomach gas.
— Sources: American College of Gastroenterology