The bicarb will react with stomach acid and produce carbon dioxide gas, obviously in excess to create the swelling.
You sure about the gas type produced? :? Humans take bicarb (an alkaline) to neutralize excess stomach acids and gases for relief. In the days before Alka Seltzer, Tums, Immodium and whatever, you simply spooned some baking soda in water and when it bubbled, it was 'bottoms up!' If you don't already have stomach gas, it can create some, especially in the different kinds of digestive systems in animals which can't break the wind for relief. And yes, Munty, it is rather painful for them.