Little Robyn, once in a while over the years I have also had exactly the experience you describe, and because there was no headache involved, at first I didn't associate it with any kind of migraine. You are the first other person I have come across who has experienced this. Those angular rainbow prisms were exactly as you describe. The first time it happened, I found it rather scary, but on the rare occasions since then it's been OK now I know that it goes soon enough - and, I think, by relaxing and "sitting out" for a while, it goes all the quicker.
The first time it happened, I was on stage in a pantomime and I had to just keep going - not really seeing the people around me properly ! No wonder the migraine happened, actually: this was after many late nights, strenuous dance routines, stressful quick costume changes...
The other few times it's happened, I can't connect it to any particular stressful event or anything. Just one of those things...