Possibly saddle too low. Similarly, too high sometimes gives people pain on the back side of the knee.

But, the change in day length could also be an issue. You've been putting in decent week-to-week miles, but if you're mostly doing 30-mile days, 90-mile days are going to do different things to you. You should always do at least occasional days that will be harder than (or at least similar to) your touring days before going on tour, to make sure you can take it.