If you are following the recommended post-op plan, things will work out. There are a few possibilities as to what would cause such fluctuation, but there is little doubt your body is burning fat. The scale, this early in the journey, is influenced a lot by water weight both leaving and retaining, especially during the first month. I doubt you could comfortably eat enough to actually put on fat weight unless you were eating constantly high calorie foods, which is extremely unlikely.
Stick closely to the recommended nutrition and make sure you are hydrating very well. Things will come together in time. If you feel you may be taking in too much salt, then perhaps you are, that could influence some water retention, but the best thing to do is drink, drink, drink.
Try to choose whole foods (phase appropriate, of course) and stay away from artificial sweeteners and ultra processed foods.