Why does our weight remain stall for several days, then suddenly drop?

Every couple weeks, something weird happens with my weight loss. I'll be losing, losing, losing, then at random, my weight lose will stall and I'll fluctuate between 2 - 3 lbs. Then, it's like my body picks the lower of the fluctuating weights, remains there for several days and suddenly it'll drop and I'll find myself 2 - 3 lbs lighter than my lower weight.

For example: All last week I kept fluctuating between 175.3 - 177.8 lbs. I thought i had broken my sleeve. On Sunday night I was 175.3 and my weight stayed there until tonight, when I got on the scale and found that I weigh 173.4.

I'm super stoked, but I am trying to figure out what's going on and if it's something I can control. At first I thought it came from being less active (last week was my Spring Break, so I went to New Orleans with girlfriends. When I left home I weighed 175.6. In New Orleans, even though we did A LOT of walking, I also ate a lot of junk, so I don't think the walking helped much. I caught a cold during the trip, so when I returned home Wednesday night I pretty much slept and laid around until I felt better on Friday. When I got on the scale my weight kept fluctuating between 175.3 to 177.8 lbs)

HOWEVER, I don't think it's the lack of exercise because during a regular week (during which time I work at a HUGE, two story school and do a lot of walking, plus I stand up all day and walk around my classroom of six year olds) and every couple weeks the exact same thing happens where I fluctuate, then remain stagnant at the same weight day and night and suddenly drop a few.

What's up with that?