Why do I have gas?

Everyone has gas. Burping or passing gas through the rectum is normal. Because it is embarrassing to burp or pass gas, many people believe they pass gas too often or have too much gas. It is rare for a person to have too much gas.

Most of the time gas is odorless. The odor comes from sulfur made by bacteria in the large intestine. Sometimes gas causes bloating and pain. Not everyone has these symptoms. How much gas the body makes and how sensitive a person is to gas in the large intestine have an effect on how uncomfortable having gas is.

source: http://digestive.niddk.nih.gov/ddiseases/pubs/gas_ez/index.htm