ETAP - a.k.a. Stich, Side-ache, Side-stich &etc - is an extremely common lateral abdominal pain triggered by exercise - especially running.

It can be severe enough to cause athletes to drop out of races etc.

The cause is unknown.

Exercise-related transient abdominal pain (ETAP) , colloquially referred to as ‘stitch’ and ‘side ache’ , is a common condition observed in many sporting activities. References to the pain can even be found in the works of Shakespeare and Pliny the Elder. Despite its widespread occurrence, ETAP has historically received little research attention and hence has been poorly understood.."

Source : Exercise-Related Transient Abdominal Pain (ETAP)Sports Med. 2015; 45(1): 23–35.

There are many proposed mechanisms for its cause, but none has yet been generally accepted as an explanation. Possible factors include :

Ischemia of the diaphragm; stress on the supportive visceral ligaments that attach the abdominal organs to the diaphragm; gastrointestinal ischemia or distension; cramping of the abdominal musculature; ischemic pain resulting from compression of the celiac artery by the median arcuate ligament; aggravation of the spinal nerves; and irritation of the parietal peritoneum."

[Source as above]