I concur with Tony, the symptoms very exactly match what I had for the first time (and only time so far) a couple of months ago. I didn't go and see a doctor, as after an hours reading up on various healthcare sites I satisfied myself that what I had experienced was visual migraine.

It was incredibly scary while it was happening though, after 30 minutes lay down it cleared.

I guess if it happened again I would probably go see a doctor, but I am not concerned about it. Notably I had been working like crazy for a couple of weeks before hand, 80+ hours a week in front of the PC.