as its coming and going sounds like SVT. mine does the same particularly at night feels like sledgehammer in my chest
As a non-professional, it's impossible for us to determine what you might be experiencing. "Hard and fast", how fast is it? The next time you have an episode, count the beats. This is very important! Also, does it begin and stop abruptly, or does it increase and dedrease over a period of time. Keeping track of this, writing it down in a log with dates, time, duration, etc. will help your physician in diagnosing your condition. Good luck!