can cause ST/T changes - simple lab test can clear the doubt. It is not common in otherwise healthy person to have electrolyte imbalance...
A normal echo is reassuring
ST-T changes highly suggest ischemia
if they are down-sloping and present at 80 millisecond (2 small squares)
If as you say the resting ECG is otherwise normal, go for an exercise ECG (under supervision) - both you and your doctor will be reassured! You are still young; have no risk factors and the possibilty of ischemic heart disease
is not common...