Assuming this is not angina
, which should be ruled out by a face to face encounter with a doctor, this is most likely a common "runner's cramp" and is due to a combination of increased oxygen
requirements of the tissues and muscles in the area during running and increased by products of metabolism
, mostly lactic acid. The problem is usually worse if running shortly after a meal. Improved cardiovascular conditioning usually improves the symptoms, as does slowing down or resting.
Dr. J. Schwartz.