i find any swings that big, either way, are water based. your electrolytes could be off, and jsut drinking water may not help and could possibly make it worse.
have you cut out either artifical sweetners, creatine or sodium recently? if i eat something salty (i normally pretty low soduim diet) i can put on 3 or 4 pounds overnight, esp if its pizza along with a coke or 2. so i think the same can be true the other way, when i cut out sweetners, creatine and cut my carbs i dropped a good chunk of weight pretty quick.
if it persists at this pace, i would worry, all other things being equal, the weight loss should slow down to a normal (well normal for you, what ever that may be).