A cardiac stress test is a medical test performed to evaluate arterial blood flow to (and indirectly the amount of oxygen that will reach) the myocardium (heart muscle) during physical exercise, compared to blood flow while at rest. A 11 MET exercise is very good and it correlated to heart rate (120-age)...not blood pressure. Because you are a smoker I am surprised you can do 11 METs.
An EKG is not a very good test for any condition other than arrhythmia and an Echo does not include blood flow to tissues. Stress test abnormalities (positive results) are an indication of imbalances of relative blood flow to the left ventricular muscle tissue. This is important because the left ventricle of the heart performs the greatest amount of work involved in pumping blood around the body. However, stress testing does not detect blood flow imbalances within the other three heart chambers.
It is usually only high-grade stenoses (severe narrowing) of the larger coronary arteries that can be detected. Severe stenoses, those of greater than 75% occlusion, are usually the result of advanced atherosclerosis (hardening of the arteries, or arterial disease) and these high-grade stenoses are the usual cause for both "stable" or reproducible exercise-related angina (chest pain) and for positive stress tests.
Less severe stenoses are automatically compensated for by vasodilation (widening or relaxing) of the ventricular arterioles during exercise and do not usually produce enough of an imbalance of relative blood flow to be detectable by stress test methods. That may or may not be what is happening, but it is a consideration.
Of interest for anyone with chest is risk of a heart attack notwithstanding chest pains: Your doctor knows the major limitation of the stress test approach is that it requires high-grade stenosis to indicate heart attack risk. And high-grade stenosis, while a good indicator of advanced arterial disease, is NOT the major cause of myocardial infarctions (heart attack). For more information on the subject you can google the COURAGE trial, which demonstrated that "intensive pharmacologic therapy and lifestyle intervention" produced BETTER survival and QUALITY of life than invasive interventions such as angioplasty. As far a blockages go, the 64 slice CT has been shown to be highly accurate in excluding coronary artery disease. In other words, if the CT shows no blockage, then there probably isn't any. But there may be other things that your doctors are looking.
The 64-CT scanner picks up as much as 98 percent of the heart's arterial network (and lacks good images for only 2 percent.). So there is a low probability of any occlusions, but the doctor may be prescribing aspirin as a safe guard in the event something was missed.