WOW! I just had one of these occur. My second one this year:
https://en.wikipedia.org/wiki/Scintillating_scotoma
https://en.wikipedia.org/wiki/File:Scintillating_zigzag_scotoma.gif
During my lunch break at work today, I went pretty hard on a flight of 200 stairs with a pretty steep incline, 5 sets, and it was also a pretty nice and sunny day. I'm not sure whether I overexerted myself, or if it was the bright sun.
The other time this happened this year was in the spring when the NHL playoffs were on, lol. I drove to my parents house to watch a game and there was a low sun as it was spring at the time. While watching the game on my dad's OLED TV (The same one as I presented here: https://ledstrain.org/d/175-oled-tv-same-issues/91), one of these started to happen halfway in. However, the sun was setting at the time and as very bright over the horizon out of my peripheral vision. I thought the OLED TV had done it, since the jury was still kind of out on it, but I didn't have any LED exposure today. Also, I am usually able to watch a 3 hour sporting event without much difficulty on the OLED TV.
What's strange about these phenomenon is I don't get any headache or ill-effects afterwards. The "attack" is pretty intense and lasts about 20-30 minutes, and then it spreads to the corners and ends. The "Signs and symptoms" describes the issue exactly. I had this very bright flashing squirmy wormy in my central vision after coming back from my Stairmeggon lunch break and grabbing my lunch bag out of the fridge. It was very intense at first, then started to spread to the corners, almost like spreading butter on bread. Once it had migrated more to the corners, my central vision was clear again, and I could read again, but then the peripheral was out. At it's worst point, I'm almost completely blind in one eye for a few minutes (left eye) until it spreads out of the way.
If I recall correctly, I've had these my whole life, even since a kid, but I just thought they were normal and that everyone got them. I must say, although a bit scary, they are one heck of a fascinating phenomenon! Just another oddity to throw into the enigma!