I don't think you can draw any conclusion from weighing 2 days in. If you lost much it's probably mostly whatever you ate the days before that was still making its journey through your metabolic system and water.

Even though you only drink water during the fast you do suffer from some osmosis because you additionally slowly deplete yourself of minerals that get excreted through urine.

If you do a one week fast you will lose weight, there is simply not enough glucose floating around for that time so you must rely on body fat to provide you with necessary energy.

If you want to know how much weight you lost it's better to weigh 2-3 days after you stopped the fast to get a good idea.