I don't get migraines, but I used to get the migraine aura you described, where your vision gets strange. It can also make you have slurred speech, feel weak, and dizziness. Sound familiar? Yeah, those are the symptoms of a stroke. So before I knew what it was, I got very scared. Once, we finally went to the emergency room of the hospital. After seeing two specialists, they told me what it really was: essentially harmless!
But strangely, since then, it hasn't really happened. What also used to cause it was playing WoW excessively, especially in purple areas. My sight would just suddenly remove a part of it. It would fill it in with colours from around the area I can't see. It's a strange feeling. I remember looking at my friend and he suddenly didn't have a mouth.