What would cause my heart to pound rapidly when reasting?

Many possibilities: There are many possible causes of abnormally fast heartbeat, some medical and some psychological (e.g., anxiety or panic disorder). If it isn't clearly associated with being upset or anxious, you need a medical examination.

Rapid heart: The most important thing to do is be evaluated by a family doctor or cardiologist. There're many diseases that can cause your heart rate increase out of nowhere. If nothing is found after all the traditional studies which could include an ekg, echocardiogram, blood test, and a very long and detailed history. Then we can start thinking about psychological causes. Don't speculate it makes it worse.

Several conditions: A heightened sensation and fast heart beats from an adrenergic state (fear, anxiety, deconditining, obesity, anemia, dehydration, stimulants such as excessive caffeine, overactive thyroid). Rhytm abnormalities such as premature beats, fast heart beats from upper chambers (supraventricular tachycardia) or lower chambers (ventricular tachycardia) are other causes. Heart monitor helps.