What is a heart scan.?

I had byp surgery 6 months ago. Am currently in a cardiac rehab program. I had my stress test after 3 months of cardio went from 4 mets to 11 mets however they are now sending me for a heart scan because my heart indicated something wasn't quite right although I wasn't getting any feelings of angina. I'm a little confused because cardio increased my exersice which is great for me. Anyway my question is if I felt fine for the stress test what is going on with my heart. And what exactly is a heart scan.