Arrhythmias and Holter Monitor Test

I am 21, female, in general good health. However, lately my heart has been bothering me. I'm afraid I have some sort of condition or arrhythmia. Sometimes I will get the heart thumps or flutters, and my heartbeats feel very heavy a lot. Sometimes I feel a quick series of beats, I can't tell if it's my heart of some muscle twitching or spasm. It takes my breath away at times and I'm short of breath and it causes me to be extremely anxious. Also, usually after exertion, my heart beats super fast upon inhalation and slows way down upon exhalation and feels out of rhythm. I'm not sure if it's only anxiety or something more. I have had several ekgs and nothing has been found, but usually the palpitations have not been active during the ekg exam. Are ekg's effective if palpatations are not present at the time of exam? If I get a holter moniter test, will this rule out any conditions or arrhythmias? Thanks!