I don't know about it 1st hand, but I was reading an article the other day about plantar fascitis (sp?) and that's kinda what it sounds like. Article said its not really the muscle, but the connective tissue in the bottom of your foot that runs the length. It gets inflamed and stiffens or contracts and OW!!!! Article said its often worse in the morning b/c it tightens up at night. Best best is lots of stretching and rest. It also said they can make a brace of some sort that keeps your foot stretched out at night, but it's not all the comfy. Try googling it or maybe web md.
Nan