Why do we get a stitch when running?

The side stitch, technically called exercise-related transient abdominal pain, is a sharp or cramping ache usually felt just below the ribs on one side during running or other jarring exercise. While its exact cause is still debated, the leading theory blames irritation of the peritoneum, the double-layered membrane lining the abdominal cavity, as organs jostle and the membrane stretches during movement. Eating a large meal or drinking sugary fluids shortly before exercise makes stitches more likely, as does shallow, rapid breathing. Poor posture and weak core muscles may also contribute. Most stitches fade quickly once you slow down, breathe deeply, or press on the area, and they tend to become less frequent as fitness improves.