What could be the reason for double heartbeats?

Hi doctor,

I am a 23-year-old male. A few years ago, I have had heart beats skipping but never took it seriously and did not consider it at all. This year I had some chest pain which happened more than once. I went to ask a doctor. He made blood tests, all the tests came back normal. Later on, I had unexplained heart attack symptoms (chest-crushing pain extended to my left arm and shoulder, a little bit of shortness of breath). So I called the ambulance. They did ECG and found ST elevation. They took me to the hospital and did blood tests but it all came back normal so later on that night I was dismissed. I visited my doctor again he made a stress examination, and ECG found rare double beats with some extra volts. I can feel double beats after stress on the heart. But recently I am having extreme random fatigue. I feel constantly dizzy, hard to stay up, hard to keep my focus, cannot do my daily tasks (walking, and going to work normally). Sometimes I get pain again randomly (chest pain, stabbing left shoulder and sometimes right jaw pain, and nausea). Please help. Thank you.