One energy drink three weeks ago should not have damaged your heart. I would see a doctor. I never drank one of those energy drinks or read the ingredients in it, are you sure that caffeine is the only thing whats in there to boost your energy?
Three weeks ago is to long to have these symptoms from an energy drink. But what do I know I'm not a doctor, but I'd go see one if I were you.
There are people who cannot take regular stress tests on the thread mill because of physical handicaps etc. and these people are given a shot instead which speeds their heart up tremendously. A friend of mine who has bad arthritis in her knees could not take the regular stress test and was given that shot she said she thought she was going to die from this rapid, pounding heart, and she had no lasting effect, she was fine not to long after her heart slowed down again.
You had these symptoms to long. I'd see a doctor and I would definately stay away from energy drinks.
Good Luck !
Caffeine is a well known to trigger heart arrhythmias. It certainly did that to me on several occasions, and that was only with coffee. Most energy drinks are loaded with caffeine.