From Christine Luff,
Your Guide to Running & Jogging.
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A. A "side stitch" is a sharp, intense pain under the lower edge of the ribcage caused by a muscle spasm of the diaphragm. They're common in novice runners who tend to breathe more quickly and shallow.

Cause of Side Stitches

What exactly causes them? When we inhale, our lungs fill with air, which press the diaphragm downward. When we exhale, the diaphragm moves up. If you have some trapped air below the diaphragm, if you've eaten right before running, or if you start running too vigorously, your diaphragm may cramp, causing pain under the rib cage on your right side.

Prevention of Side Stitches

If you tend to suffer from side stitches, try to avoid eating within one hour of running. Make sure that you always warm up before your runs.

Running in extreme cold temperatures may induce side stitches, as it's less comfortable to take in deep lungs full of frigid air until you're thoroughly warmed up. If you're running in cold weather, try breathing through a scarf or neck warmer.

Treatment of Side Stitches

To get rid of a side stitch, try altering your breathing pattern. Take a deep breath in as quickly as you can, to force the diaphragm down. Hold your breath for a couple of seconds and then forcibly exhale through pursed lips.

If you get a cramp in the middle of a race, you might want to try changing your breathing/striding pattern. If you always exhale when your right foot strikes the ground, try exhaling with the left foot strike.

If all else fails, you may have to stop and walk briskly for a few seconds while concentrating on deep breathing. Continue running after the stitch goes away.