What happened to my vision?!

What happened to my vision?

I am working on my computer and I get a sudden blind spot just to the southwest of center where everything seems scrambled. I can see the color of whatever is in the blind spot but it looks buzzy. So I figure it is a floater. I close one eye to see which eye this is in, then I close the other. And I realize the pattern is exactly the same! So I figure, OK, this is going on in my brain. Over the next ten minutes I notice the blind spot sort of expand into a rough vague donut shape. After a while it fades away- total time15-20 minutes. My vision is back to normal. I looked up all possible diseases that affect the eye and found nothing that fits the bill. I have had problems with allergies this past week and possibly a cold too. I figure I probably blew my nose too hard and dislodged or moved something in my brain, where it proceeded to affect my visual center. It is all I can think of. This happened once before but much more briefly. Any ideas?

Not to worry. You have ocular migraine/ ophthalmic migraine - this is a variant of migraine without headache. It is not serious at all.
No one knows the exact cause but it is thought to be due to spasm of blood vessels in the brain (in your case, the part of the brain connected with eyes). It does not cause any permanent damage - but it is unpredictable and u can have this anytime again in future. If you are lucky, you may never have this again in your lifetime - if it recurs, you will have to wait 15-20 minutes for it to pass off.
If you need any further information, leave a comment in your question and I will check it again in 2 days.
P.S: Your sysmptoms do not suggest detached retina at all - but no harm in having it checked by an Ophhalmologist to have it ruled out.