What causes stomach pain?

Many things: There are many, many things from every facet of medicine that can cause stomach pain. Is there a particular one you are inquiring about?

Many: Most people think their whole abdomen is their stomach, but the stomach is in the upper middle or upper left of the abdomen. If your lower abdomen hurts, that could be your skin, muscles, bladder, uterus, or intestines. If the upper part of your abdomen hurts, it could be acid reflux, pancreatitis, or other problems. See a doctor for an assessment.

Many possibilities: A person with tummy pain between the belly button and the bottom of the rib cage can have a variety of problems. A doctor's examination is needed, unless the person is young and healthy, and the pain is mild and resolves quickly. Possible causes include: stomach acid problems, reflux symptoms, pancreatitis, gall stone related problems, abdominal muscle strains, some sort of hernia, etc...