Sorry if I'm repeating what others have said here, I don't currently have time to read through the thread fully.
A stress fracture doesn't normally crop up all of a sudden like that. There are usually overuse symptoms present for a while before things progress to a stress fracture.
My guess (heavy emphasis on the guess part) is that your calves were tight from hill ride, that would limit the flexibility of your ankle so when you were working on the steep slope you likely over flexed through your mid to fore foot to compensate and may have sprained or subluxed a joint there.
Try
rolling your calves using a foam roller or rolling pin and then roll your feet on a tennis ball or similar. Self massage the bottom of your foot and push up through the sole of your foot with one hand while flexing your toes downward (as if you're trying to pick something up with your toes) both actively and with your other hand.
Try doing it several times during the day and see if that helps. Even if you had a stress fracture, this self treatment will not cause you any harm so it's worth a shot.
I hope that helps.