Having just had a migraine end, why can’t I have this, rather than pain and vomiting?

They're not the same thing, although visual migraines can appear at the start of a "real" migraine. It's a distortion of vision with glitchy geometric patterns that pulsate and move a bit. Here are some attempts to recreate them: https://duckduckgo.com/?t=ffab&q=visual+migraine+&ia=images&...