hi nicholas, well done for riding those distances, I can only dream about that distance!
But to add my thoughts to your post. Are you using genuine chamois? Most modern nix are synthetic, in past years genuine chamois was best softened before use with 'softeners'. But synthetic chamois doesn't require this. BUT I have heard that even now some distance riders are using 'personal lubricant' ie KY jelly to reduce saddle friction. Hey! don't laugh, it would be heaps better than vasoline- because vaso' will shorten the life of your chamois, plus not work as good as KY or other such cream.
Plus if you only ride those distances infrequently then your butt needs time to harden.
Even experienced riders get sore after a layoff or even if its a day when we sweat more than usual problems can happen.
regards, sillystorm