Well, maybe it was my change of socks?
I changed two factors: sock setup and shoes. So no way to know which was the cause, but I'm going wading in a river for a mile this weekend and will use my old sock set up.
There wasn't any fungal problem. I got home, aired my feet out, and they returned to normal the next day. As far as I understand it, this is what happens: wet feet leads to maceration, which makes skin crack and makes it *more prone* to fungal or bacterial infection. Fungal being more common because it grows in moist areas of skin. So the maceration isn't caused by fungus. They are both caused by water. It wasn't cold at all the day I hiked. It was warm. Very strange, as I've hiked in all day wet weather, colder than this, many times with no problems whatsoever.