What Causes Constant Flickering Blind Spots In Vision?

I am having constant flickering blind spots in my field of vision. I am a medical student, and have been constantly stressed, studying 8 - 10 hours/day and having 3 hours sleep. I am 20 years old and have been suddenly seeing these spots. I was thoroughly checked out by my optometrist and my vision was checked thoroughly and is fine. ia m having unbearable depression from this. Nothing wrong with my health. I am very healthy and have had no health issues.