I'm 15 and have just started a cross country program.
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.