What is the cause/what are the causes of stomach acid?

Natural: Your stomach normally produces hydrochloric acid through a proton pump in the parietal cell.

Many things: Many people have trouble with gastritis or reflux, due to hyperacidity or making too much acid. It can be due to a medical problem like zollinger-ellison syndrome, but most are due to the food you eat, or when you eat it. Numerous medications will help with it. Talk to your doctor about it.