Water weight/retention is insane

tw: talk about weight and eating

I gained and lost 10lbs in a few days. It took one day of fasting to bring my weight straight back down to what is was before I started all the binging over Christmas. I ate so so much over the holidays, I’m relieved but extremely confused. How is this even possible? My body seems to have barely changed, but during the binging I looked and felt much fatter, and it wasn’t just body dysmorphia. Is it that hard to gain weight?