Originally Answered: Did I just lose water weight this week?
it can be if you used to eat lots of salt and suddenly stopped or cut way back.
Also, look to what time of the month it is for you.
When I actively diet, the week after my period is super good, I am in the zone, dropping the pounds, etc. The week before is another story, either no weight lost at all or a slight gain :(.
Your weight lose will always fluctuate somewhat depending on what you ate, your activity level, etc.
I have a friend who actually gained weight the first time she tried dieting but she was working out so strenuously that the muscle she gained canceled out any fat she lost and while she looked slimmer, she weighed more. Freaked her out!