Just my opinion...but I would try a different pair of shorts...even if you did wear the same pair on both rides. I sometimes have issues with shorts. For example...today I did a 70 mile ride wearing a pair of Shebeest SSS shorts. I kept having this weird scratching sensation on my girly parts all throughout the ride. I wore the same type of shorts (I have 2 pairs) on a 73 mile ride that I did on Saturday with absolutely NO issues whatsoever. Go figure.
I've also been having a LOT of chamois issues with ALL of my PI shorts. Never really had any problems with them, until recently...now they all bother me for some odd reason.
Try a different pair of shorts.
2012 Seven Axiom SL - Specialized Ruby SL 155