I would be making an appointment with a Neurologist. It sounds like a complex migraine, but always best to get things of this nature checked out. Do you ever get a migraine before or after the symptoms? I get migraine complications to the point of stroke like symptoms (left side paralysis, facial droop, and slurred speech) typically within 24 hours of having a migraine. A friend of mine gets the loss of vision, light headed, dizzy, tired, & nauseous with hers and she doesn't always feel the migraine at all. Other people are known to see an aura around people and things during an "episode". Depending on the migraine type you may be able to take medication to prevent the onset or something to make it go away. In my case I have a regimen of vitamins I have to take to keep everything on track.
The primary etiology is intermittant blockage of a blood vessel providing sustenance to the optic nerve. Could be a clot. Could be a spasm. You might never find out what it is.
In any event, the drill see a physician immediately. They will likely order a CT or an MRI, possibly an MRA.
Evaluation is done by an opthmalogical neurologist.