You won't know for sure. What you need to do is let go of that worry and understand what all your body is going through right now. You had 80% of your stomach removed. Ok, think about it like this. Think about the hardest you've ever fell or been hit by something. Think of the deep bone level bruise you got. How long does it take to fully surface? A week? 10 days? Well, that's internal bleeding, coagulation, blood rising to the surface, your white blood cells flooding to the area, more blood to begin healing, the area is hot for weeks! The bruise will raise and take weeks to slowly heal and go away. Can be 6 weeks before the bruise disappears. And that's just from a blunt force trauma.
Now imagine you have 80% of your stomach removed, you have hundreds of sutures/staples in there, you have 5-7 incisions points - each the equivalent to being stabbed by a knife. Now think about all the swelling and healing that requires? Think about all the processes in the body, the shock, the attempts to heal itself. Our bodies will retain water, send blood and fluid to the areas that need to heal, it will slow down the metabolism in order to promote healing and prevent weight loss as that is a primal process in our bodies.
There is a lot of science behind why people don't lose or gain in the first few weeks post surgery and most of it is related to how the body behaves as it's healing.
Don't waste another thought or worry on this!!!!!!!! Just focus on meeting your goals, I PROMISE you, you will begin dropping weight like crazy if you follow your post surgical stages and protocols. You are doing great, it's so hard and scary, I had a ton of these types of questions and only learned these answers from asking. I am so proud of all of us, TBH, we are SUCH badasses for doing this. You are gonna do great and you are totally OK.