I wouln't worry about that at all. In MMA, it wasn't uncommon for guys i trained with and myself, the odd occasion, to drop as much as 20 pounds in 24-36 hours for weigh-in. That was all water, of course.
What you describe is nothing to be concerned about.
Also - try weighing yourself each morning, afternoon and just before bed. You might be suprised by the fluctuation in weight that occurs throughout a normal day.
I can vary +/- 5 lbs within a given week - depending on my training cycle/program, etc.
I am familiar with the weight fluctuations throught the day.
For a long time now I have been weighing myself weekly every Monday upon waking up and after going to the toilet. I never saw such drastic changes in body weight before.
How the hell do you drop 20 pounds in 36 hours?