What Causes Dizziness?

i have had an MRI, blood work, a cat scan, EKG, and all checked out fine. started having dizziness about 6 months ago. started out not so bad and only came on at the end of a double shift. before 2 weeks ago i worked 2 jobs for 4 years, one day shift and one night shift. about 70 hours a week. two weeks ago a bad dizzy spell came on after a full 8 hours sleep. that's what got me to go to the doctors. after being off work 2 weeks don't have as bad a dizzy attacks but sometimes i have to grab my balance cuz i feel motions when i am not moving. also my gagging has gotten worst. plus i am feeling a little anious sometimes were i never felt that way before. what could this be?