The most likely cause is a vitreous floater or small speck of debris which floats in the vitreous gel and casts a small shadow on the retina. People with floaters sometimes say, it is like seeing a small bug just out of the corner of their eye and then it disappears when they go to look at it. Please see an ophthalmologist if you have new or different floaters or flashes of light, pain, or loss of vision. Again, this is just my best guess but I cannot tell exactly without seeing you in person.
I recommend that you seek the care of an eyeMD and have your retina checked. Once in a while, this may represent a retinal detachment or floater, due to posterior vitreous detachment. If you experience a sudden increase in the number of floaters, a peripheral curtain or a flashing light, you require immediate treatment.
Sandy T. Feldman, M.D., M.S.
ClearView Eye and Laser Medical Center
San Diego, California
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