Same for me—had a couple out of the blue maybe ten years ago, none since. It freaked me out, and yes it was like I was looking through a prism or kaleidoscope, and I kept checking my contacts. Called optometrist and they said it was an ocular migraine, but they said it was not necessarily cause for any alarm if they went away soonish. Odd as I don’t have issues with migraine headaches. best wishes