Could also (and more likely - depending on your age) be vitreous detachment. (Forming of "floaters")

But I have been advised by my ophthalmologist that any time you see flashes you should see an eye doctor as soon as possible. Flashes and spots are also symptoms of retinal detachment, which can cause permanent blindness.

Unrelated to my PN, I also have a corneal disease that will eventually require me to have some form of corneal transplant surgery - so I'm all too familiar with eye doctors......