Sometimes I see things that aren't there, but I'm not on drugs, mentally ill, depressed?

Last night I went to my bedroom and I turned on the light. Very briefly I saw a purple flash in the lower corner of my eye. I thought it was a problem with the light so I turned it off, rubbed my eyes and stepped out. I tried to recreate it by walking in and quickly turning on the light but it didn't work. There weren't street lights turned on or anything outside the window. I thought it was the reflection of the box springs wheel so I sat down and I tried looking at it with the light on from different angles. Nothing. This isn't the first time stuff like this has happened. I will just be sitting watching tv and I'll see a white flash in my right eye. What is this? I'm not on any medication, I'm not mentally ill. I can't read out of my right eye and the doctor said my body is only using the left eye (unless I close my left eye) What is this? If I'm going / am blind in my right eye why am I seeing things?