Alice -
Ketosis is simply your body using fats for energy instead of carbs (glucose). Your body can run on fat byproducts called ketones when glucose is in short demand. On SBD, you may or may not be in ketosis, it actually doesn't matter. Regardless, as you cut carbs, your body will burn up your stored glycogen (carbs) which carry water molecules with it. That's why many people lose 8-13 lbs the first week or so.

At your already low weight you probably have a low BMR (basal metabolic rate) so unfortunately you may have to watch calories as well. You're doing great so far. Don't go by scale weight alone, go by how clothes fit.