I had some issues with semi-sudden onset of problems with a new bike once. What happened to me was the seat quick-release binder stopped binding all that well. I'd hit a large pot-hole and my rear would hammer the seat down a little bit. It wasn't enough for me to really notice the height change immediately, but I quickly started getting sore in places where I hadn't been sore before, though not where one might suspect... Anyway, the point is that in addition to making sure the bike fits, make sure it continues to fit.