Why do we get a stitch in our side when running?

The exact cause of exercise-related transient abdominal pain, commonly called a side stitch, is still debated. The leading theory is that friction or tension on the peritoneum, the membrane lining the abdominal cavity, causes the sharp pain. Running, with its repetitive bouncing, may jostle organs and stretch the peritoneal ligaments. Eating or drinking large amounts before exercise increases the likelihood because a full stomach is heavier and pulls on these ligaments. Another theory involves reduced blood flow to the diaphragm. Stitches tend to decrease with fitness. Slowing down, pressing on the painful spot and taking deep, controlled breaths usually help.