If someone is not used to drinking lots of water, and they were eating more sodium before, they'll tend to drop a whole bunch of water weight in the beginning. It seems like sometimes starting to work out (while drinking lots of water) will get the water weight moving, too, but I'm just making that up based on past experience, I've got no scientific evidence.
It's a great sign that you've been taking good care of your body!
Congrats on the loss.