72 hours of not feeding myself was a dreary commonplace when I was a teenager. I absolutely can go days at a time without eating. It got me down to 137 from 237 in the space of four months. It also makes me light-headed and despondent. I've pretty much gotten used to being dizzy and despairing by 4pm. Eating enough to not be that way means I gain a pound a day. When I was riding to work I had to time dinner such that I actually had enough available calories to make it home without seeing stars. And still I lost no weight.
I know you mean well, but "worth getting to the bottom of it" means four doctor visits' worth of "surely it's because you're an idiot." if it's fluid, that means that my metabolism depends on me sweating out approximately a cup of water a day in order to function correctly. It would also mean the weight would go as quickly as it comes. If I have three slices of pizza and a beer, the scale will have a number 2 1/2 lbs higher the next day. Doesn't matter if I biked two hours beforehand; if I want it to go back down I have to eat sparingly and exercise strenuously for two weeks. That's assuming my routine doesn't already include strenuous exercise, in which case, forget it. It's just there. The summer I spent working out, running 30 miles a week and hiking 8 miles per weekend added 30 lbs to my frame that didn't come off for ten years because it came on through exercise and then never left.