Our expert says:
That's an interesting problem, because it's limited to only running. The two things that jump out at me initially are a) that it might be an asthma of sorts, and the rhythm of running is what causes it to happen. Swimming is a lot easier becuase the breathing pattern is dictated by the stroke, and cycling allows breathing in any rhythm, whereas running has impact that affects it. Exercise associated asthma is relatively common and may be in play for you.
The other option is possibly of more concern, though the fact that you're cycling and swimming OK makes me less concerned, and that's that your chest pain is something to worry about. The only way to know if it's really something to be concerned about is to have an ECG and diagnose it.
However, I suspect maybe something a lot more mundane and that's that the impact is causing pain when running. Is there any chance you've bruised or damaged ribs recently? Even a slight dislocation, and the extra effort of breathing plus the landing every half a second adds up to chest pain. So that's a third option. Obviously, safety first says check out the ECG, make sure it's really nothing, and then consider further assessment.
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