Exhaustion

Why does every muscle in my body ache? Over this weekend, I never made it to the gym (A went to the one in our building) or to do C25K, but managed a little (very little) weightlifting and dance. Most of the weekend's "activity" was walking - over 21 miles for both days according to my fitbit - but does walking really make the body this tired? Is this some sort of fatigue? So tired that I keep forgetting to set my fitbit's sleep mode, which isn't even that difficult to do. What's wrong with me?