From my understanding, it's a blood circulation problem. In heavy exercise without proper/ sufficient warm-up, the blood goes to four limbs rapidly, internal organs detects the abnormal decrease of blood and issues a warning signal, then you feel the diaphragm pain.
The only solution is to slow down for a while and let the blood flow resume normal. To prevent the problem, make sure you have enough warm-up exercise or increase the intensity of your exercise slowly.