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You are probably referring to a posterior vitreous detachment. The vitreous gel in the posterior part of the eye liquifies with age and shrinks. As it shrinks, it may cause traction on the retina which may lead to flashes of light and floaters in the eye. Usually, it goes uneventful, but sometimes it amy cause a retinal tear which can lead to retinal detachment and serious loss of vision. So, a person with these symptoms must get a dilated fundus examination to rule out any retinal tearwhich you have already done and so you can relax. 53 years of age is not all too young for posterior vitreous detachment. It can occur at a much younger age. Your flu shot is not likely to have anything with this. At least no such association is known. These symptoms will get better with time as you get used to it. Thanks.
Numbness and pressue is not related to it. Get a refraction done. Pressure could be related to it. You can use a lubricating eye drop like Refresh Tears 3 times a day. For numness you need to ask your doctor. An examination is necessary to find that out. Thanks.