I have this too, very scary and frustrating.
This forum is not about "exactly". Even in person physicians can often not tell exactly what the cause is. Without examining you all we can do is suggest likely possiblities.
Use the search feature and archives to read about "eye migraines" "Ocular migraines" and TIA and transient ischemic attacks. That will cover the possibilities.
You need to see an Eye MD find one at www.aao.org
JCH MD
I also have the typical visual disturbance associated with migraines.
Please, let me know what exactly this visual disturbance is?
From just a statistical standpoint the most common thing that would cause that is a completely painless ophthalmic migraine.
JCH MD