Your body is constantly adjusting how much water it has for lots of reasons. It's almost impossible to pin it down to one thing. Maybe it's the sodium, maybe is hormonal, maybe you are fighting an infection, maybe you are allergic to something in the sauerkraut, maybe Mars and Jupiter are in the wrong alignment.

I'm up four pounds in the last three days despite averaging a 2450 kcal/day deficit for over a week (so should have lost over 5 lb).

While it could, conceivably, be a sign of something significant, it is almost certainly just a transient fluctuation that will resolve itself in time. Hope you start feeling better soon.

Hang in there.