why do you post in the cold & flu community did it happen during or after you caught a flu?
do you have other symptoms, balance, memory issues, numbness, tingling ?
It could also be caused by Blood Pressure issues, does it happen more often when you stand up quickly? could be from Orthostatic Hypotension
someone else asked a similar question
http://www.medhelp.org/posts/Eye-Care/Silver-Spots-in-Visions/show/224434