For about a month I have felt my heart flutter and at that very instant it is as if my breath is taken away. It is so fast that it is not uncomfortable but it makes me very worried. I feel it mostly in the afternoon and evening. I have never had anything like this; infact, I have never really felt my heart flutter. I had run a 10K run last November 2009 and they did a 20 minute heart test with all these suction cups on my body and an ultra sound machine next to me. After, they said my heart was fine and great to run the race. I did run and there wasn t a problem. I took this report, given to me by the Cardiologist for the run to a new doctor (Cardiologist) near my home and he listened to my heart, looked at the report and said my heart is great. He advised me to check my Thyroid. What could this be? I am 40, eat well, non-smoker and exercise on a regular basis. Thank you for any advise. Millie