Hello and hope you are doing well.
If your X rays and lung function tests were normal, your symptoms of breathlessness is unlikely to be due to lung disorders. it could be due to cardiac disorders. In cardiac disorders the heart is unable to pump the blood adequately, more so when subjected to stress like exercise. Certain systemic disorders like anemia, diabetes, vitamin deficiencies and thyroid disorders can cause these symptoms. So please discuss this with your doctor, who may ask for an EKG or a treadmill test to rule out a cardiac cause for the breathlessness. He may even ask for blood tests to rule out other causes for your symptoms.
Hope this helped and do keep us posted.
The burning may be acid reflux was it relived by an antacid?I think GERD may affect breathing also, we have a great GERD forum on MH take a look and put your question there.It may also be allergies .