What happened to my heart?!

yea the same thing happens to me (only thing different is ur ONE yr older then me haha) I don't think it's anything to worry about cause it happens if you either have rlly good hearing or you could of gotten rlly calm and you would hear the heart beat in your ears, I try to ignore it and think of something else or else i go crazy listening to th-thump for hours.

myself, own expiriance

The pounding in your ears stopped, or your heart stopped for a moment?
If your heart is beating irregularly, it's entirely possible you have a clot in your veins. My brother had a heart clot when he was only 5. Anyways, tell your parents about it, you might want to get it checked out by a professional instead of us amateurs. It could save your life. Or it could be nothing, who knows.

Maybe h thought you hears a noise from heart, I get noises like thuds in there and I know sometimes it's my ears like a blockage not heart.