Hi!
I know what you are going through. I, too, suffered that missing heart-beat that would take my breath away. You are suffering stress so that could be a cause. I cut out all my caffeine-no coffee no soda no tea..I have had not one irregular heart-beat since then. The scary thing is sometimes when the heart flutters it can send a clot and that's how certain strokes happen.
I notice that you wrote you just had a "hard" workout. Do you have a cooling down period? You know after,maybe, fifteen minutes of intense workout you slowly come down a bit. When you are working out hard,sometimes, the heart is going at 170 beats per minute..That's fast and combine that with anxiety and you might find the cause. So you need to slowly bring the heart down-that means no slumping or sitting so keep moving around (walking) many heart attacks happen during a workout. Many people don't know about this
i suggest buying a watch that monitors your heart beat when you are working out..Most workout stores carry them. Your gym should have your heart range during workouts-that means the safe range for your heart during workouts.
Watch your diet and maybe cut out the caffeine-it raises the blood pressure and heart-rate.
I know that scary feeling-drink tons of water and maybe try mediation. It's been over three months and I feel better!! If it continues go to your doctor and try a stress test there.
You will be fine