Medically speaking, the "side stitches" that you experience are
called "exercise-related transient abdominal pains" or ETAPs.
These stitches are very common and not serious, although they
can be painful and can put a crimp in your exercise session.
(People who exercise are much more likely than couch potatoes to
experience ETAPs.) Stitches are cramps or spasms in the
diaphragm, the muscle that separates your abdominal organs from
your chest and lungs. When you inhale, your diaphragm drops
down; when you exhale, it expands upwards. But vigorous movement
can jerk the diaphragm down when it is expanding upward, leading
to a "stitch." You may have noticed that stitches usually occur
on your right side - this is because the diaphragm is attached
to the liver, which is on the right. To prevent stitches, you
can try the following tricks: - Breathe evenly and deeply, not
shallowly, using your diaphragm. You're breathing correctly for
exercise if you can see or feel your belly expand when you
inhale. - Avoid running d