Thank you for writing!
Caffeine can cause the heart to beat a bit faster due to the fact that it raises your blood level of epinephrine/adrenaline. Those who drink copious cups of coffee should consider cutting down. It's interesting that this also happened to you while drinking decaf. Remember, however, that decaf still contains a little caffeine.
It may be that you feel your heart beating rapidly without or without caffeine. But caffeine is also found in foods like chocolate, breakfast foods and headache medications. Check your diet.
The only way to know whether the caffeine is causing your heart rate to be faster is to quit it altogether. Give your body about a week or two to adjust and see how you feel. If you feel that your heartbeat is still elevated, call your doctor for advice.