It probably is just water. When you cut grains you often loose a lot of water weight in the first couple of days. Remember to salt you food, with the water salt and minerals get flushed out.
If the loosing proceeds after a week or 3, just count calories for a week or so to check how much you are eating. You don't have to continue counting, just to get a better picture of how much you are eating and the fat/protein/carb ratio