I know, logically, that a gallon of water weighs around 8lb so shouldn't have been so surprised. But I hurt my back and have been just walking for cardio and doing yoga for a couple of weeks, no lifting, no running. Diet I estimate is about the same, and my weight dropped by 4lb over the first few days and has retreated slightly more from there so now I am down by 5lb. I am pretty sure this is water, I don't think one can lose muscle this fast, and I'm eating well, do not think it could be fat loss either.
Reassuring in a way, if it jumps 5lb when I jump back in I won't freak out.