I'm not a doctor by any means, but this kind of sounds like an ocular migraine. I get these now and then. For me they can come on either slow or fast, and most of the time go away after a fairly short time, it some times help if you can just close your eyes for a while and rest. I know they can be real scary, some times I get them so bad that I can hardly see, but again they do go away after a while and I feel fine, but for the headache that allmost alway follow, not a bad headache, but one that is just there. Anyway if this sounds like what you have, the good news is that they are harmless, but never the less scary the first few times it happen. With that said, you might what to ask a doctor about it, but I'm put money on it that it is an ocular migraine. Hope this helps
Reagards Ken.