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.