What Causes Dizziness With Sickness?

Hi I am 46, had a C5/6 spinal fusion, over 2 years ago, I have had two bouts of intense dizziness in which I have struggled to get out of bed and have been sick. Both times after doing more than I should have. I am now finding I am constantly dizzy, mainly when moving my head or turning over in bed, I feel like I am close to being sick, as it is worse today, I am trying to get to the bottom of it. I did go to my Doc after the last bad episode but he says it wont be from my fusion? Thanks D