Hello,
First, I would make an appointment with your regular physician for an examination and to discuss with him/her about your situation. I would keep a record of when you are experiencing this problem and how long it is lasting. Secondly, discuss with your primary physician about a referral to see an opthalmologist. From your description of the situation, I believe it is possibly vascular.
Secondly,
You would benefit from a consult with an opthamological neurologist, a sub-specialty. My suspicians are it is glucose-level related, because that is the one thing that changes after exercise.