No reason to be alarmed or concerned...as Lisa said it's mostly water. As you rehydrate you may gain some of it back, or maybe not. Generally after surgery we slip in to ketosis for a while which increases urine volume (not necessarily frequency) and in general even if we are keeping up on hydration requirements, we are still a bit dehydrated for the first couple weeks or so. This is why some people have a little three week-ish stall because they start rehydrating which counteracts some of the weight loss, but not everyone experiences the stall.
Anyway, no reason to be concerned, just keep following the recommended post surgery guidelines and try to stay as hydrated as possible.