I've just been experiencing this for myself this week, exactly as Travis13 has initially described. It lasted for about 30 minutes.
While searching for answers on the net, and found info related to: it may be caused by stress, anxiety, too little sleep, sleeping in a way to put bad pressure on my eye, floaters, allergies, etc., it spurred on thoughts of my week.
I work in a high stress atmosphere plus lots going on at home right now. I haven't had a lot of sleep lately with being very busy, while still trying to maintain some sort of fitness. However, I wake up tired. Although I am prone to eye allergies, I give myself a jump start with 1-2 cups of morning coffee.
After my second cup, i was still feeling thirsty, but lots happening at work, so didn't do anything for about 1 hour. Then... the SPOTS hit me. I thought I was looking at the computer screen for too long, but they got worse, and i felt a bit nautious. AND, i was thirsty. I started drinking water - 2 cups! So, I know I was dehydrated.
After about 20 minutes after my 2nd glass of water, the spots/blurred vision began to dissipate. The nausea feeling has also left.
I don't know if all of my symptoms are related, but could the blurred vision, the flashing spots, all be a result of dehydration & too much caffiene? Possibly... but wouldn't a doctor be able to test for this or do they just sometimes miss the obvious?