It sounds like you may be having a rhythm problem - not a structural problem. Rhythm problems don't cause murmurs
or any other sorts of noises. Go back to your doctor (or another doctor) and ask about wearing a heart monitor for a day or two. Basically you'll have a few electrodes attached to your chest and you'll carry this little monitor around for a day or two. Some monitors continuously record your heart rate
and rhythm and can detect abnormalities. Some monitors have a button you push when you feel an episode. When you push the button, it begins recording AND goes back several minutes as well.
Without knowing anything about you, it sounds like you may have Atrial Fibrillation
- which is fairly common. It's when the top chambers of your heart begin beating faster than they should and become out of synch with the bottom part of your heart. It's typically asymptomatic (90-95% of the time), but you might be in the minority. The list of causes in a young person is long. It's typically quite harmless - just uncomfortable for some people. There are medicines that can help prevent the episodes, but it may just be your diet, caffeine or any number of easy fixes. You should be able to go back to your primary care
doctor for this.