The EKG didn't show a heart attack, doesn't show ischemia (blocked vessels), and an enlarged heart can rule out many serious heart coditions. But the EKG has many false negatives so the doctor wants you to have a stress test to be more assured.
You may electrolytic imbalance, thyroid disorder and many other posiblilities. About 4 years ago I went to ER for shortness of breath, and the doctor very quickly knew that I had a heart disorder and treated!