What Causes Heart Fluttering And Dizziness While Having Mechanical Aortic Valve?

I have experienced on and off heart fluttering over the last couple of weeks. Had some dizziness once or twice. I think it may be caused by caffeine and/or stress, perhaps low blood sugar. I reduced my caffeine intake. This occurrence has happened twice before in my lifetime and it eventually went away. The first Cardiologist did check me out and prescribed a beta blocker and the second Dr. checked me out with a monitor and told me that it will probably would go away and it did. I have a mechanical aortic valve and take warfarin.