The avid exercising could contribute to the stomach discomfort if you happen to be exercising on a full stomach or do stomach crunches. Both can help stomach acid splash back onto the esophagus if that LES valve is not functioning properly.
Whenever someone experiences tightness of chest and shortness of breath, I suggest consulting with a medical doctor. You could be experiencing exercise induced asthma or a heart symptom. Both should be investigated, and treated if need be. The types of symptoms you describe can stem from one or more than one source.
Rose