I am not a doctor, but You may be overbreathing, or experiencing pacs or pvcs (heart flutters). You should be able to tell what it is, and if you think its your heart--you can have it checked out. Its usually benign and very very common. (I have pvcs myself). :)
I would agree...why not go get checked out by your doctor, but anxiety can cause a myriad of symptoms to include what you describe. In my experience with this, we tend to breathe fast and shallow and my stomach can be tied up in knots for a very long time. Have you had a history of anxiety? Have you been going through a lot of stress lately?
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