pounding heart

I'm 19 and for the past month or 2, my heart has been feeling like it's pounding really hard, and sometimes i feel dizzy. This feeling almost always happens only at night when I'm laying down. And when I think about it more it makes it worse. I have gone to the ER a few times but everything looks normal to them. But I know obviously something is going on. I'm on a 30 day heart monitor now but I dont know if it will be able to detect anything because it's not like my heart is racing really fast, it's just pounding really hard and im starting to feel a really light pressure, probably from me being so tense and worried about it. Any idea what it could possibly be?