What happened? It is a simple chemical reaction between an acid (vinegar contains acetic acid) and a base (baking soda, also known as sodium bicarbonate.) When mixed, there is a chemical reaction that releases carbon dioxide, and this is what causes all the bubbling and frothing. The base neutralizes the acid, and it is through the chemical process of that neutralization that gas is released.
       This is just what happens when some people swallow baking soda to counteract excessive acid in the stomach. Many people will notice that it triggers gas and belching, with relief of their symptoms to follow. It is not that the baking soda causes the release of previously formed gas somehow locked in the stomach, but the gas is a sign that the baking soda is working, and it is the neutralization, or buffering of the stomach acids, that brings relief.
       (Are you sure it's baking powder you're eating? Baking soda would make sense, but baking powder is a mixture of baking soda and an acidic powder; if you ate it, it would largely neutralize itself without doing much for your stomach acid.)
       Dyspepsia and heartburn are two words used to describe the symptoms of excessive acid in the stomach and esophagus. With dyspepsia, one feels a burning, aching or gnawing discomfort in the upper abdomen, relieved by antacids such as baking soda, Tums or Rolaids, and also relieved by food. Food puts the acid to work by occupying it with the process of digestion rather than letting it lie around in an empty stomach, irritating the lining. If the acid washes up into the lower food tube (the esophagus), then heartburn results. This will be a gas-like sensation or burning in the mid-chest region. Some heartburn is bad enough to cause an acid taste in the back of the mouth. Once again, antacids will temporarily relieve the symptoms.
       Small doses of baking soda are perfectly safe. Indeed, our bodies already contain large amounts of the same chemical, which is used to buffer the acids generated by the body's metabolic processes. However, you can take too much, and excessive use leads to a shift in the body's acid composition. If you are someone with other health problems such as chronic diarrhea, lung disease or kidney failure, the effects are more challenging to the body and more difficult to counteract, and could lead to more severe symptoms of what is called alkalosis, or an over-correction of acid content in the body.
       Two spoonfuls a day is not really excessive for most people. However, there are better approaches. Other antacids, such as Tums, Rolaids, Mylanta and others, are often more effective.
What's more, there are medications, some available over-the-counter such as Zantac, Pepcid and Tagamet, that turn off acid production by the stomach and bring more complete and sustained relief. By prescription, there are even stronger medications such as Prilosec and Prevacid that are the most potent medications of all, virtually shutting down acid production and very useful for refractory cases. Your doctor has probably discussed some of these with you, and if you can afford them, they are definitely worth a try.