Dr. Anne Marie Albano in Prevention Magazine

"Gas is created when the acid in your stomach is churning while nervousness runs throughout your body," says Anne Marie Albano, PhD, associate professor of medical psychology at Columbia University Medical Center. Another culprit: swallowing too much air. As your stress levels increase, so does your rate of breathing—taking in extra air means you have to expel it at a quicker rate, too.
 http://www.prevention.com/mind-body/weird-anxiety-side-effects