have you done an excessive amount of walking or put any stress on it prior to the pain starting? is it possible it's a stress fracture? i'd go to the GP and get it checked - the fact the pain is escalating would concern me enough to check it out.

it's possible you've just gotten a chill in it if you're in a cold area (my feet are ouchies now cos it's cold this morning) but the increasing pain just doesn't sound right