Whats wrong with my brain?

I have always had dizziness problems, but never thought into it. Last week I got dizzy, (standing up) and hit my head on a door knob. Earlier today, i stood up , touched the door knob and I got dizzy , and i was starting to faint, then started shaking? or vibrating violently, for a good 30 seconds. I need to know from a doctor whats going on.