Your diet may not have changed, but the way you eat probably has. You swallow big mouthfuls, and as they travel down your esophagus, they push the air there into the stomach. As the wad of mouthful passes the sphincter at the top of the stomach, the sphincter closes again, and trap the air in your stomach. As more food going in, or as you move about, the pressure inside you stomach rises above the level the sphincter can hold, and the air is flushed up the esophagus again. We all eat some air with food, but small amounts stay in and travel with the food down, only to break out later as the bowel movements increase pressure on the other end. And in some cultures, Turkey - for example- belching by guests is considered an honor for the host. Why? It means that the food is so good, the the guests pack their stomachs to the fullest!