Unusual Chest Pain.?

After I run for a while or do any type of cardio for a length of time, Below and to the left of my sternum starts having sharp pains. The pains last between 5seconds to 2 or 3 minutes. What could it be?
Best answer:
I have experienced that but only under stress, it hurts when I take in a deep breath called pleurisy little air openings in the lung tissue, very painful sharp stabbing maybe, mine go away, if this is what you are feeling and it goes on go to a dr, and maybe get a chest x-ray..
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