What Causes Sudden Fading Of Vision Causing Difficulty To Focus?

I felt like I was going to pass out. But it was different. not like the spinning or going to throw up kinda pass out. Suddenly my eyes went to tunnel vision and I felt very wierd. I was driving at the time so I slowed down and tried hard to focus. it faded just about as fast as it came. It came back a second time but not nearly as strong. so I pulled over, got out and walked around for a minute. My head still feels wierd.