just water can easily change your weight +/- 5-6 lbs over the course of a single day. i have been hovering at a 24 BMI for about 4 months now. a few days ago i made a really big pot of soup. because so much of what i have been eating is liquid, i weighted myself toward the end of the day yesterday and then again first thing this morning. i dropped 5 lbs overnight merely from getting rid of all that water weight.
when i was on my major loss streak (-50 lbs over 5 months), i kept track of my weight and logged it at least 3x a day. in any one day my weight varied, but consistently over weeks and months, it gradually went down at the rate of about 2-2.5 lbs/week. every few days i would see a new low and eventually there were also highs i would see for the last time.