I am a double board certified clinical doctor and can help you with this question.
There are some main possibilities.
Heart disease. Part of that has been ruled out, but you need a chemical stress test to get the full picture.
Lung disease. Pulmonary function testing needs to be done. This will show if there is a chronic problem.
Small clots to the lung. This is often missed. It can relate to a clot in the leg. You need a high resolution CT scan of the lungs to see if this is the case.
Bronchospasm from acid reflux. Often missed.
I would take this list to your doctor and go over it carefully.
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