What Causes Dizziness Along With Loss Of Consciousness?

On my way to work I suddenly blacked out and fainted. As a result of that I fell severely damaging my lips and busting a tooth. I went to the ER and they did a cat scan and MRI on my brain to determine what caused me to fall. It came back normal. I have been experiencing weakness, dizzy spells and headaches. Any idea what this could be?