Hi, I Am A 64 Year Old Male With Mitral

Hi, I am a 64 year old male with mitral valve prolapse that is hard to detect using a stethoscope. I have been experiencing heart palpitations frequently in the last few months. The palpitations are most severe when I am lying down or reclining, and least severe when I am riding my stationary exercise bicycle. I had a Holter monitor attached for 24 hours 2 years ago--also for palpitations--but these seem to be more severe. What could be causing this? How would I know if I have atrial fibrillation? Thanks!