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The symptoms you describe are termed photopsias. Photopsias, along with floaters (black or grey spots), can be signs of an impending retinal detachment. This is the reason the doctor examined your eyes carefully. It sounds like he performed indirect ophthalmoscopy with scleral depression. This test is performed to look for tears on the retina (the back of the eye). Fortunately, he did not find any tears or a retinal detachment.
The photopsias occur as the jelly that fills the eye separates itself from the retina. This causes traction on the retina. The retinal cells interpret all stimuli as lights, which explains your symptoms. As long as the vitreous separates itself without causing a tear, there is nothing to be concerned about. Once the vitreous separates completely, the flashing lights will stop.
I would recommend that you return to the ophthalmologist in about one month to once again be sure that no retinal tears have occured. If you would like to obtain a second opinion then you should go ahead. I always encourage patients to do this since a fresh look at the situation can only help.
I hope this helps and that your symptoms soon resolve...
It is normal for the vitreous to separate from the retina...and yes it does happen to everyone of us as we get older. This will not have any negative effects on your eye health or your vision, provided a retinal detachment does not occur.