Silent migraines. That's what they're called because usually no pain with them but just as you described. Mine start as a dot or a blur spot where I can't see definition of things but can still see. Zig zag of colors moves left to right for me. Used to be, I'd lose my peripheral vision but not so much anymore. Stress! It's caused by stress. No meds taken. Used to have several a week..now rarely. If you can, close your eyes till it passes.