It happens all the time -- to my husband, to my daughter, to friends -- they go out and celebrate with their friends and family, then the next morning they step on the scale and exclaim that they've put on WHOLE POUNDS. How could this possibly have happened?!
Having observed this phenomenon for pushing-a-half-century myself, the short answer is "it ain't fat." People don't put on fat that fast -- i suspect it's physically impossible to store adipose tissue at such a rate even if one is actually trying to do it. For years i would have said it's salt-induced water retention, then in my late-twenties and early-thirties (when i first began reading about how dieting works) i would have said that when we store or take carbohydrate out of storage, we can get big swings in our weight, because every gram of carb is tucked away with three or four grams of water which is either tied up or freed up very easily. These two factoids, along with the realization of monthly swings of water retention, were behind my relaxed attitude toward several-pound fluctuations in weight for decades. One is "officially" one's lowest weight (never measured except first thing in the morning, after peeing), but that higher weight was at least acknowledged.
When i first went on Atkins around a decade ago, my weight steadied out in a way it NEVER had before; he explained it in terms that were just an elaboration of what i already knew. When i went out for a restaurant meal and ended up bloated, it just HAD to be about the salt, or maybe there were insidiously hidden carbs.... It wasn't till i moved on to ultra-low-carb primal-style eating that i suspected there was something more to it. I had switched to sea-salt and i wasn't afraid to use it, because the explanation came about insulin itself causing water retention ... and besides, i didn't notice deleterious effects of generous usage at home. It was only eating out that made me all puffy like the bad old days! So maybe it wasn't actually SALT -- maybe it was another sodium-based compound? i asked myself but hadn't really begun to explore when...
it ain't the salt, and it ain't the sodium per se (although the sodium benzoate and sodium sulfite might be contributors). That nasty bloated puffy feeling one gets after even the best-quality and most-innocent-seeming restaurant meal is all about the histamine load ... which is "always" higher away from home because of how food has to be handled in an industrial setting. Eat a "fresh" food at home, and an identical one in a commercial establishment, and the latter is bound to give you a bigger jolt of the old bioamines.
Authorities on the subject tell us that eating ANYTHING -- even thinking about eating -- is enough to start our bodies producing endogenous histamines. They're neurotransmitters, after all, which regulate a stunning array of normal bodily processes ... like digestion and appetite. It's an unfortunate coincidence that allergic problems use the same pathways and we begin to associate histamine-response to pathological-histamine-response (just like we do with physiological vs pathological insulin resistance)....
But despite the care restaurant kitchens have to take for the sake of good hygiene -- or even because of it (think "preservatives") -- it's common to come away from one of their meals toting a bigger histamine response than one gets at home. They're likely to use high-histamine ingredients like wheat flour, bean-gums, tomato-paste and spices in such a way that most home-cooks don't.
So when you feel waterlogged and horrified by the scale's reading after what you THOUGHT was a pretty innocent restaurant meal, just remember that your body is just trying to protect you -- those distended and more-permeable veins all over your body are making every effort to get blood and white cells where they're needed. They don't care if you feel puffy and uncomfortable; they're trying to warn you that there are ingredients in the aged cheese or preserved meat (etc) that your body finds a bit toxic. Add stress, heat-and-humidity, red wine or beer, and anchovies, and your pizza might turn into the kind of perfect storm that will make you REALLY sick.