I have not read this thread and don't have time now to go through it, but ...

I had a similar experience after a ride 50 years ago, New England, month of March, my first long ride on my racing bike. It was Wednesday. Saturday I rode the opening frostbite series race. Had to drop out from real knee pain. A doc at the race diagnosed me as having chrondomalacia patellae (CP). Told me it was for life, that there were things I could do and outlined them. Took several phone calls later to further my recover and progress back to racing that season. Years later, for a previous forum I was on, I wrote a long post about that experience and what I have learned since. (It was 20 years before I ever saw all of what that doc told me in 1977. Thank you, Dr. Kish.)

I've posted that post here several times. Do a search for knee and my username or patella/ae. (I may have mis-spelled chrondomalacia and I still am not clear where the "r" belongs.). It's a long post but I have never seen all of what I wrote in one place.

And to the wisdom passed down from that doc? I raced that season; my last for other reasons. Rode 5k miles per year for the next two decades. After a 6 year marriage of little riding, ramped up to 7k per year. About 200k after my diagnosis, over half on fix gears. There is cycling after CP but it does have to be managed. I played by that doc''s rules and at 72, I still have me OEM knees.

Edit: CP can come on with ideal seat height. It can be triggered by bike changes, building bike conditioning too fast, cold weather and hamstring tightness on a bike that fits perfectly. My seat height was dead on. Approved by both the doc who also raced and the race promoter, an older Belgian fellow who grew up in the heart of the sport.