possibly there is an old injury in your leg that your body was working on healing while you were fasting. Loosie is a member of this forum who experiences something very similar to this... her leg starts to hurt a few days into her fasts and the pain gets so bad she has to stop.
it's common to experience pain/flare-ups in the parts of the body that are undergoing and needing the most healing. i had a very bad flare-up of eczema on my face and hands (where i typically previously used to get it) during my last fast. your body sort of goes backwards in time, reversing all the old injuries/illnesses.