So after two weeks of a 2600 calorie or so a day lifestyle change (this isn't a diet, it's a permanent lifestyle change), I lose 10lbs.
I go on a business trip and have three days of 3000 calories a day plus 500-1000 calories of excercise, and I "gain" 10lbs.
Well no not really; what happened was two fold:
1. my acid balance, and hormone balance were thrown off by the change, and by the fact that I am in a UC flareup, and as such I am retaining a lot more water. I'd guess that at least 90% of that "weight gain" is actually excess water retention. I'm feeling thirsty even after drinking 1-2 gallons of water a day, and that's really just a screwed up brain signal.
2. My body has been kicked into fat storage mode by the rapid reduction in caloric intake; and is reducing my basal burn because it believes we are approaching a famine. So its not like I'm eating extra; it's that my body has decided that it needs to burn less.
Why do I know it's not really me gaining back the weight? Because I was at 405 5 days ago, and then two days later, I "gained" 5 lbs overnight. Then a few days later I "gained" another 5lbs overnight.
That's not actually possible given the caloric intake I've had. In fact, I would have to have had zero burn in the last fix days to gain even 3.5 pounds, given what I'd eaten. I've eaten a total of about 12,500 calories in the five days since I weighed in at 405, which is 3.6 lbs of fat equivalent. In that time I've also excercised about 3300 calories worth.
So, 9,200 calories of effective consumption; or a little more than 2.6lbs; presuming I'd burned NOTHING basally, which is obviously impossible.
When the body goes into famine mode, it can cut your basal burn in HALF, (though that's a bit extreme). You'd note it with poor circulation, reduced heart and respiratory rates, joint and muscle aches, headaches, reduced energy and fatigue, excessive sleepiness, and reduced body temperature; and to some extent I have noted some these things, but I'm sure my basal rate isn't quite THAT low at the moment.
This by the way is why you never cut your caloric intake by more than half, because then your body goes into EXTREME famine mode, and it actually starts shutting things down on you; not only do you not lose weight, but you can get health problems.
So lets assume five days at HALF my basal rate, or around 2200 calories per day.
Hmm, funny, that's more than my effective calorie consumption. Even at half my basal burn rate, given my caloric intake and excercise, I should have lost 1/3rd pound.
Now, I'm willing to believe that my caloric intake and excercise calculations are off by 10%, or even 20%; so lets say that my actual caloric intake was as high as 17,500 calories. I've burned at the absolute minimum to maintain life, 11,000 calories, and let's take excercise out of the equation entirely.
That's still only two pounds of weight gain even POSSIBLE FROM THE LAWS OF PHYSICS in five days. Which of course means that, as I said, 90+% of this "gain" is excess water retention.
In a couple weeks my hormone levels will adjust, my basal burn will settle out again, my water retention will drop to normal, and I'll start losing again. This is a pretty typical cycle, though three weeks is awfully fast for it to start; it usually takes 4-6 weeks.