Re: Breathless feeling...
Did they give you a shot of radioactive "dye" before, during or after the test to show any areas of the heart that were ischemic (starved for nutrients.)
If they did not, an ECG alone is not conclusive of "no ischemia."
Plain vanilla stress and echo can be passed by many people who would fail the radioactive markker test or an angiogram.
Does the breathlessness come on with hard exercise and go away after 5 minutes rest? That kind of response is a sign of angina. Any pain or discomfort mid-chest with exercise?