Hello, Welcome to healthcare magic! The symptoms you have reported seem to point to suddden diminution of vision which needs to be attended soon. Had it been fall in glucose level, it should have improved after food. The reasons can be anything ranging from impaired blood supply to the retina or visual area of brain, vitreous bleeding, etc. if you are a diabetic, it puts you at a higher risk of such complications. Your age, associated health conditions, medications, presence of pain in the eyes and many other details do matter a lot to arrive at a diagnosis. Hence it is suggested that you meet a doctor as early as possible. Take care! Get well soon!