What is the reason for pain in lower right side of your back and stomach?

Anything. 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.