What Causes Palpitations Along With Dizziness?

I am a 26 yr old white male, I drive a eighteen wheeler for a living and for the past month I ve been experiencing my heart beating faster light headed and dizziness, along with headaches almost like migraines. I went to the ER the other night and they did a EKG and it came back clear, ER Dr referred me to a clinic where they have did another EKG, heart Echo and a Caratid on my neck. Echo and EKG are clear no results on Carotid yet. They sent me to have a holtor monitor placed for 48 hours will be finished with that on Friday morning. My question is if all the test come back clear what could be going on for all of my symptoms? I am scared of being on the road and this keeps happening whether I m on the road or at home. I don t want to end up running off the road and injuring myself or others because I nearly pass out with all the symptoms I m experiencing. Thank you for your time. Chris