There is a lot of research on the absorption of water by nylon. It is supposed to expand with water absorption and also water will act as a lubricant (plasticizer) between molecules allowing a string to stretch further. A thin strip of nylon under tension is used as a sensor for simple humidistats. So I'm not sure why your strings are going sharp, but perhaps your ambient temperature is lower now than in the summer? Perhaps the change in temperature now, when you play, is larger than the summer time change????
I don't know what happens to fluorocarbon.
"When I was younger, I could remember almost everything, whether it happened or not." Mark Twain