At the risk of being an exhibitionist (again), I did this little experiment a couple years ago:
http://www.3fatchicks.com/forum/showthread.php?t=51927
I actually got less variation than I thought I would.
Up, down, up, down it goes. We are all something like 70% water and those small (and sometimes not so small--I gain 10 lbs after a race), short variations are usually it. The goal is to manage body fat and the first thing that pops up is water. Extra fluid in the digestive tract, in sore muscles, everywhere.
Having said that, it sucks, and right now I'm up (within normal fluctuation) and yeah, it sucks.
Stick with it.
Anne