While I don't think anything serious is going on with you at such a young age, I'd say you need to see a cardiologist as soon as possible. (I'm under the impression that you only saw a regular primary care physician.) If it WAS a cardiologist that you saw, I'd say try not to worry so much about your symptoms. Sometimes when we worry about our symptoms during the physical exercise or whatever that we're doing, it makes it worse. If anyone really concentrates on their breathing, they'll start feeling 'funny'.
Sooooo, being in the medical field, I would say you need to see a cardiologist and have a stress test done since this occurs when you're exercising. You'd be put on a treadmill while they watch your pulse, blood pressure, etc.
Keep me posted.....Take care!
Chronic pain: 6-12-92 MVA, L5-S1,2,3 Radiculopathy, Fibromyalgia, Chronic Myofascial Pain, (Have a Spinal Cord Stimulator in place, too.) Bursitis. Medications I take are: Oxycontin, Oxy IR, Neurontin, Robaxin and GoLytely.