Water retention?

So I’ve lost around 100 pounds, one thing that has always been strange to me. Is that when I would splurge for like five days and eat like crap the scale would say I gained like 20 pounds, and another strange thing is that every single time I would eat like crap after doing really great with fasting and working out I would wake up that morning, and I would look in the mirror and swear I look fatter and it’s weird. It might be a bit of body dysmorphia, but I’m currently on vacation and I saw a picture of myself when I first got here five days ago and then I took a picture of myself today and I look heavier in my face and body and stuff.