As others have said, it sounds like the aura phase of a migraine - google 'scintillating scotoma'. I used to get this regularly (2-3 times a month), sometimes when my blood sugar was low but other times there wasn't an obvious explanation. I had an MRI scan (all clear) then went on medication (beta blockers) to try and stop it happening but didn't like the side effects so decided to just live with them instead. I have no idea if this is related, and the doctor doesn't seem to know either, but about 18 months ago I discovered I was anaemic and started taking iron tablets. Since then, I get the visual thing about once every 3 months if that.
I get the visual disturbances without the pain afterwards, just a bit of nausea. I get them about 2-3 times a month, usually when I'm tired or hungry. I was referred to a consultant and had an MRI scan which was clear. The consultant didn't seem too concerned about them and said it was up to me whether I wanted to take medication to try and stop them - apparently a short course of medication is sometimes enough to 'flick a switch' that stops them (it sounded a bit more scientific the way he put it...) So I went on medication without realising it was beta blockers and the effect on my running was awful - like running through treacle every time - so I decided to stop. I don't drive otherwise I'd be more concerned about them - they're annoying but bearable.