blow, as always , answers to this topic are completely person related, but who knows, could it have been soap and not rinsing out enough? As previously brought up, small seat position and / or bike short and /or specific seat issues can all be the cause of it.
Also of course not having done so many hours riding before, as long days riding will always tend to show up small issues that didnt show up on shorter rides, just as long loaded rides will show up position issues vis a vis muscles, knees etc as we are working so much harder on a fully loaded bike than unloaded.
realistically, you are going to have to methodically try diff changes to see what and where the issue was.
Ive ridden for many years and now have both seats and padded shorts that work for me and dont have any issues like this even riding long days, day after day, even for months--but I am aware than improper cleaning, an issue with my shorts or seat, can always have the possibility of having a saddle sore (thankfully very few experiences with this personally).
diff bike shorts with diff padding fit diff people differently, and I might as well bring this up, do not wear underwear under padded bike shorts.
diff seats and slight position changes affect diff peoples bottoms differently, so there is no end to suggestions of fixes, not internet easily fixed answers here alas.
good luck with figuring out what it was, as mentioned, often its just a little thing--one reason that when one has a setup that works, bike, seat, bars position, padded shorts etc etc that works, one doesnt screw around with diff settings once you find the variables that work for you and all your comfort.