According to WebMD,
Side cramp or ''stitch": This cramp strikes you in the side, as the name implies, or even in the lower abdominal area. It's mainly the result of shallow breathing, not breathing deeply from the lower lung, says Jeff Galloway, a 1972 Olympian. He's a veteran runner who has trained more than 200,000 runners and walkers and runs a marathon-training program. ''The side pain is a little alarm" alerting you about your breathing, Galloway says. An imbalance of blood electrolytes (such as calcium, potassium, and sodium) in your body may also contribute, says Pete McCall, an exercise physiologist and spokesman for the American Council on Exercise.