Side stitches are caused by a stretching of the ligaments that run downward from the diaphragm to hold up the liver. You breathe once for each two strides. You breathe out when one foot, usually the right, strikes the ground. So, your diaphragm goes up when the force of your foot strike causes your liver to go down. This stretches the ligaments to cause pain.
You can relieve the discomfort by stopping running and pressing your fingers deep into your liver to raise it up toward your diaphragm. At the same time, purse your lips and blow out as hard as you can against the tightly held lips. Pushing the liver up releases the stretched ligaments. Breathing out hard against resistance lowers your diaphragm. The pain usually goes away immediately and you can resume running.