This is a strange symptom and I must say that I am hearing it for the first time though I am quite an experienced ophthalmologist. If the eye examination was fine then it should not be a cause of concern. Do ensure that the retina was examined. At your age, you are expected to have developed posterior vitreous detachment. This is a normal ageing change. The vitreous gel in the posterior part of the eye gets separated from the retina and shrinks. This may cause floaters and flashes of light in your vision. Because of exercise, may be the shrunked vitreous comes infront of the affected eye and causes a huge spot. You can get a retina examination done to see if there is a vitreous detachment in that eye or not. Thanks.
Please, click the ACCEPT BUTTON so that I can get my dues. BONUS and a POSITIVE FEEDBACK will be appreciated.