It sounds like a typical aura from migraine. At least that’s what mine looks like before an attack, but they don’t necessarily have to be followed by a headache. It usually spreads across the entire field of vision before it disappears in about 20-30 minutes.
Yep, my original migraine symptom circa early 1980s. It’s called ‘scintillating scotoma’ or ‘fortification spectra’. I hate it with a passion.
To enjoy the experience all over again see here: Scintillating Scotoma - YouTube)