Side stitches are pains that occur usually just under the ribs when running. It seems that an unconditioned diaphragm is the cause of this pain more often than not. Either running a greater distance than usual or at a faster pace than usual will bring this pain on. The diaphragm is a muscle that separates the chest cavity from the abdomen. It moves down when you inhale and moves up when you exhale.
Side stitches seem to occur most often on the right side of the body.
It is possible that the liver may alter the motion of the diaphragm more on that side because of the larger right lobe. First run slower and longer. Breathe rhythmically and make sure that you are not holding your breath. You can also try counting your breaths 6 in hold 3 out with a forceful exhalation for a 4 count or whatever seems to work best for you and your running rhythm. Another breathing tactic that is tried is exhaling against resistance through pursed lips.