Hey, Runatic!
The familiar running pain has a scientific name: exercise-related transient abdominal pain, or ETAP.
Its cause is less settled. A research review considers several explanations. One of the more plausible is irritation of the parietal peritoneum, the membrane lining the abdominal cavity. Review in Sports Medicine
So “it's always a diaphragm spasm” sounds more certain than the evidence allows.
The review cites a study in which around 70% of runners reported the pain during the previous year. A very familiar sensation, with an explanation still under discussion.
Your side knows how to ask a pointed question.