Your cardiologist is probably right that you don't have a problem. Young guys like yourself can have palpatations that don't mean anything.
Stress can cause slightly irregular heart rhythms.
However, if you experience any other symptoms when your heart has these "extra beats" like light-headedness, blacking out, chest pain
, or pressure in your chest, you may want to get a second opinion
Electrophysiologists are heart doctors that specialize in the way the nerve
impulses travel through your heart.
Did your cardiologist give you and ECG and/or a stress test
, where you run on a treadmill while you're hoooked up to a monitor? Ask about these.