As you get older, as your health deteriorates, or as your body's pH levels get thrown out of balance, your body may be unable to produce proper levels of gastric acid. Gastric acid (which is what the stomach uses to break down food) is actually a bit different than hydrochloric acid. Gastric acid contains a very small percentage of hydrochloric acid and a very large amount of other elements. My theory is that when your stomach is not functioning at optimal levels, the gastric acid is being produced in lower quantities, containing a higher percentage of hydrochloric acid and a lower percentage of the other elements that should be present. Therefore, the gastric acid being produced is much more acidic, causing heartburn and improper stomach digestion that can lead to all kinds of other health problems. Many people take heartburn medications that neutralize the stomach acid (using calcium or other alkalizing elements), but this can actually just make the problem worse.
I have had the same problem and the thing that has helped me the most is taking either lemon water or apple cider vinegar mixed in water a couple times a day. Although these are acidic, they are considered to be alkalizing to the body and for me have seemed to help normalize gastric acid production.