What Causes Irregular Heart Beat?

I am a 41 year old female and for almost a year now I have been having through out the day on the left side of my chest a feeling where it feels like my heart feels like it beats a couple times in a row it is hard to explain like if you have indigestion and you lay down on your back and you feel like your heart beats a couple of times in a row. It is off and on through out the day. I have been to a Dr. and had a EKG and a tress test they said it was normal and suggested that it was anxiety. When I went on vacation last year it went away and then at the end of summer it came back it drives me crazy thinking there is something wrong with me and I worry about it everyday and I am tired of thinking about it. I quit drinking caffine too. Could you help me figure this out?