Fonty
Body pain isn't okay but it's normal enough. :) I have managed to continue exercising throughout most of my fasts and have suffered no harm from it. Some fasting supervisors strongly advocate active fasting rather than excessive rest also, claiming that more toxins are released with the activity and the sweat glands are stimulated taking more load from the kidneys and the liver.
Also it is 'natural' to be active while fasting, we generally have more energy than while fasting and how would we be able to find and gather food when hungry if we didn't have energy while fasting? No matter how long we have fasted for we would still need the energy to travel possibly miles for our first meal after hunger returned so no matter how long the fast we should still have the energy to do every day work.
Andrew.