21 yr female. heart arrhymia. test results show an irregular ekg and stress test, but a regular chest xray and echo. what could be causing it?

Get the facts: Unless we can examine your entire ekg, holter and health history, nobody here can tell you a thing. This can be anything from the trivial extrasystoles / flipflops that most young folks' hearts do up to several times per minute in health, to something serious. Caffeine is the usual suspect in trivial disease, and if your physician tells you not to worry, then try your best to comply.