It sounds like you have already had this problem fairly well worked up by your team of doctors. As you are probably aware, the most common cause of persistent flashes of light to the one side of one eye is a detached retina. It sounds like your doctors were worried about this as well because you've already had three eye exams and a CT scan
to look for it. Most likely, you have injured your retina, but in a way that is not picked up on the CT scan. Since it is not detached, this injury should heal on its own and the flashes of light should go away. The normal imaging is very reassuring. In terms of your sinus infection, this type of thing is fairly common to see on a CT scan. You should be treated with antibiotics if you have symptoms such as a headache with a very runny nose.
I suggest that you continue to follow with an ophthalmologist
. This is the best type of doctor
to follow and treat any issues with your vision going forward. It is also the type of doctor most qualified to examine your retina on a regular basis to makes sure that it appears normal. Hopefully, your vision will return to normal on its own soon. Good luck.