Fodmapa. Gas (air) taken in either from swallowing or from fermentation of sugars in the diet. The stomach and large intestine overlap in the vicinity you are referring to. Google low FODMAP diet (fermentable Oligo-Di-Monosaccharides and polyols). The most well known is lactose. Colon bacteria ferment the sugars and create gas and fluid. Gas distends the colon (sigmoid) and causes pain.