"Side stitches"

The stitch, an exercise-induced pain in the side, is a medical mystery. Many physicians call it a spasm of the diaphragm. Others say it is akin to the headache—a symptom with a multitude of potential causes. The world’s perhaps most-published authority on the stitch, Australian scientist Darren Morton, disputes both of those theories, calling it an irritation of the parietal peritoneum, the outer lining of the abdomen.

Whatever it is, Dr. Morton’s research shows that the stitch afflicts about one in five participants in a typical distance race... For some it will be debilitating, reducing them to a walk. For others it will be a pace-slowing nuisance...

Though it can feel life-threatening, the stitch doesn’t cause death or injury, sports-medicine specialists say. Unlike a sprain, torn ligament or bone fracture, the stitch doesn’t require rest or recovery. For many, the stitch is fleeting, often vanishing as training increases. Most never seek medical treatment for it...

Defined by Merriam-Webster as “a local sharp and sudden pain especially in the side,” stitch isn’t a new word. “Tonight thou shalt have cramps, side-stitches that shall pen thy breath up,” says Prospero in William Shakespeare’s “The Tempest.” In a first-century mention of the stitch, Pliny the Elder recommended treating it with “the urine of a she-goat, injected into the ears,”...

A 2000 study of his on prevalence by sport found that within the previous year the stitch had stricken 75% of swimmers, 69% of runners, 62% of horse riders, 47% of basketball players and 32% of cyclists.

If I had to bet, I would favor stitches being caused by diaphragmatic spasm induced by respiratory alkalosis.   But that's just a guess.