waking up with fast heart rate

ok this morning i woke up at about 6 am with a pounding heart.i could feel i in my chest and it was at least over 150 bpm..this happens once every few months..the heart beating is so fast and hard i can feel it and hear it in my chest and i usualy burp when it starts to calm down..it will slowly slow down but itll remain fast even when i cant feel it in my chest

ive never had an ekg done during this fast heart rate so im not sure if its an abnormal rhythym, but i have had ekgs done at other times, one time they said there was a st wave elevation that might be related to pericarditis but the cardiologist looked it over and didnt think so, plus the other ekgs ive had since were normal..
ive had two echocardiograms done and on the report it says its fine with mild trivial mitral regurtiation
ive had chest x rays and ct scans..and all the blood work up in the world..theyve checked my thyroid and checked if i was anemic..

the only times this used to happen to me i thought was when i ate at night and then laid down to sleep but this time i didnt eat for two hours before laying down..im worried this could be atrial flutter or some kind of dangerous rhythym but it happens so rarely that i dont think a holter moniter would do any good..also ive heard about sleep apnea but i dont feel like i really fit the other symptoms of it..and i dont have any nightmares that i remember either, and i really dont think its a panic attack because when i wake up im wondering what woke me up so fast, and then hear and feel my heart in my chest and i take deep breaths to try to slow it but it takes so long.

also im 21 and my blood pressure is normal

.if anyone has any idea what else this could be id appreciate some kind of answer

but other than sleep apnea is it possible its some sort of dangerous heart arrhtymia? theres so many different kinds..superventriculartachycardia atrial flutter and fibrillation ventricular fibrillation