Of course I wouldn't want to discourage the OP from having this checked out by a competent medical practitioner, but I have little doubt that this was a migraine effect with the rather beautiful name of "scintillating scotoma". I've experienced them many times.
A scotoma is a blind spot, and this is surrounded by a "scintillating" pattern of bright, iridescent light. My migraines generally start with a feeling of unjustified mild unease or sometimes unjustified mild euphoria. The visual effect starts with just the "scintillation", a tiny flickering point of light that is there whether my eyes are open or closed.
This then expands, leaving a blind spot in the middle. Once I walked into somebody at full tilt in a dimly lit corridor, unable to see them because of the scotoma. Usually however I find that I can just about carry on working on a computer while the scotoma progresses, if I have to.
I sometimes get a headache afterwards, which can linger dully for a few days, but I don't always get the headache. Taking a few paracetomol at the first signs sometimes seems to prevent the headache.
Dr Oliver Sacks, who wrote the book about Parkinson's disease on which the Film "Awakenings" was based, has also written comprehensively and entertainingly about migraine, and I would recommend his book to anybody who is interested in the subject. There is a long list of things that can cause migraines: stress, over-excitement, tiredness, oversleeping, certain foods, lack of food, and there are also interesting psychological triggers: relief from stress can trigger a migraine, and we also seem to have the power to cause them semi-voluntarily in oursleves as a way of escaping from something we dread.
Food, a sneeze, a snooze, a nice poo, vomiting and orgasm are all cited as possible methods of stopping a migraine in its tracks (probably best not to try these all at once).
