I'm just 5lbs short of having lost 100lbs in five months, but have noticed massive weight swings (up to +10lbs in a day) after cheat meals... why is this happening?

Exactly five months ago, mid-August, I decided it was high time I started getting in shape. I started at 480lbs and am currently at 385lbs, just 5lbs away from 100lbs loss- a milestone, to be sure, but still far from where I'm going; 230lbs.

While how I've accomplished this so far isn't what this thread is about, I'll just answer that now, since I'm sure some people will be wondering (feel free to skip to the second paragraph down if you're just here for my titular question):

I have been killing myself in the gym 6 x a week and adapted a whole new eating behaviour. I took up and fell in love with boxing in August... that's what started this, and what has made the working out tolerable, as it's loads of fun and a great stress-reliever... initially started going 3 x a week, but kept adding a day... until now, 6 x a week. Prior to recent Covid closures, I was seeing my boxing coach 2-3 x a week, as well as going 3-4 x a week to a gym I joined (to work more on strength & conditioning), but for the past three weeks have been stuck to working out at home, which I'm mostly fine with... thankfully I've got a punching bag, which is the bulk of my workout anyways and a fantastic source of intense cardio. But it's not the same as with my coach and at the gym, as I don't have access to all that equipment, and nobody to watch over me to make sure I'm not doing things wrong. But anyways, things should reopen within a few weeks and I'm making do just fine for the time being. As mentioned, I also adapted completely new eating habits. Prior to the dietary change, I was eating completely without consequence- whatever I wanted, whenever I wanted, zero regard for how it was literally robbing me of life. There was no semblance of regularity to my diet, so I couldn't begin to tell you what I ate in an average day... but let's put it in the 4.5-5k cal/day range. I've never been a breakfast person, so eating my first meal in the day was a regular thing for me... but night eating was a definite issue. That said, I was able to adapt to an 18:6 IF immediately with no strong cravings, and have kept that pretty strictly since beginning. I'll actually end up doing a 20:4 oftentimes, so let's say my IF fluctuates from 18:6-20:4, sometimes, inadvertently, a OMAD will end up happening as well. When I first started, I was extremely strict, not consuming any more than 1500cal/day... but that just wasn't sustainable for several reasons. Nowadays I'm much less strict and try to be in the 2-2.5k cal/day range. I also don't let IF rob me of a social life, so if I know I'll be attending some get-together where (not-the-healthiest) food will be a central thing, I'll just roll with it... There is always a consequence, closely linked to the question in my title, and that is that I'll be several pounds heavier the following day, often having to spend 3-4 days working the weight back off. It infuriates me to see all of my extremely hard work in the gym put back on in the span of one dinner, but, again, I'm not letting IF rob me of a social life- it's fine, really. But so yeah, that's how I've gotten here... I notice that on average I am 2lbs lighter after a workout (which spans from 1-2 hours depending on what I'm doing... I am completely drenched in sweat and sore as heck), and usually wake up 3lbs lighter than the night before (water?).

So the reason I'm posting today is something I was forewarned about before starting out on my journey, and something that I've been noticing happen to me for several months now. Before I started, my friend told me about a friend of his who was even heavier than me when starting out, and how his weight grateful fluctuated/swung depending on what he'd eat- sometimes up to 15lbs within 24 hours. And while my swings haven't been as extreme, they're not far off... I have logged days where I woke up 10lbs heavier after cheating the previous evening. Of course I'm not eating anywhere near that amount/weight in food, so I'm left kind of baffled as to how/why this is happening? It then takes me several days to get the weight off again, but I simply deal with it. So, where exactly is this weight coming from? I'm not eating 5 or 10lbs worth of food, so what is it? Makes no sense to me.