But the best person to tell you for sure is an eye doctor. Not the kind that measures you for glasses, but an opthalmologist who will dilate your eyes and look inside. There are many things that can be causing the symptoms you describe. One thing is an ocular migraine. It's like having a migraine in your eye. It doesn't have to cause any pain. I get migraines and see things that look like flashing lights, and bright flashing snakes, and sometimes I just get a dark block that blocks part of my vision. These images last for about 15 minutes and then they're gone. So, maybe that's what's going on with you. You should see an eye doctor to make sure though.