I Am Having Occasional Hollow Like Feelings In My Upper

I am having occasional hollow like feelings in my upper chest. They only last an instant. I do have a stent and was recently checked out including a full treadmill test with dye, and had no issues. I exercise regularly, am active and in good health. I have no issues nor do I have this feeling when I exercise, always when at rest. I have had these sensations before but they have always gone away, this time they have been occurring for about 2 weeks now.