Everybody seems to have the stitch at one time or another, but it isn’t an injury or strain. It’s a pain, mostly on the right side (although it can be on the left) that goes away when you stop your activity.
One cause of the stitch is spasm in your diaphragm. The ligament linking your diaphragm to your digestive system can become tense when playing sport and puts stress through your abdomen. Other causes are gas in the large bowl, or the liver becoming congested. The important thing to remember is that it is not harmful and there is no need to worry about the exact cause. The stitch can be worse if competing under pressure or if you suddenly start a harder training load.