An EKG measures electrical impulses, a nuclear stress test looks at blood flow to the heart, an echo looks at the physical structure of the heart/valve/chambers. The inverted T wave is not uncommon. As long as the blood flow and structure all look good I wouldn't be worried. It's kind of like a hiccup. Could be caused by electrolite imbalance, something as simple as low magnesium level. Something like this is difficult to pin down as we are all different and normal level for me may be to low or high for you. Calcium, Vitamin D, Potassium, all play a role.
Disclaimer: I am NOT a doctor. You should only accept medical advice from a licensed professional. Nothing I say should be used to influence your decissions. Batteries not included, some assembly may be required.
A stress test only gives a rough indication of heart health.
It is often used to test for ischemia.
Having no chest pain during the relatively short exercise does not mean that there is no ischemia.
A good Duke score does not rule it out either.
An ST depression gives better indication.
The rule is that 2 mm or more, significantly indicates reversible ischemia.
With 1.5mm you are a little below that, but with this depression you are still suspect for ischemia.
However, although ST depression often comes from ischemia, there may be other causes for it. Therefore your doctor ordered the echo.
The fact that this depression only happens at high exercise intensity most probably means that there is no immediate danger for you.