Too much exercise causing vision problems?

i started running three years ago when i was al fat and chubby...my body is in perfect shape now, but my vision's totally screwed up ...im just 17 and i have an army of floaters( little SOB's,aren't they??) and shitty grainy vision...i look up at sky and i see those very tiny white dots zapping throgh my vision...its worse when i do some excersize, as it feels like there is a blot of cells appearing with each heartbeat...is it something to do with running?? my tennis coach warned me that my intake of water is too less than what i sweat out, and that it can cause array of problems ..i went to an opthamologist and he did some pupil dilation and looked into the eye, only to tell me that he saw not even a single SOB and my retina was perfect and that it was all in my mind...he said further that the visual snow was some (__________) phenomena where the red blood cells moving in the eye are visible, and that EVERY PERSON IN the world sees them, but a few notice them...is it due to over excersise? or bad diet??? is there any hope in the world that one day i might get rid of these lil' ****ers?