in my experience, the most likely culprits are TOM, too much sodium (or not enough water?), and calorie intake -- since you've been exercising an hour a day, make sure you're eating *enough* calories. my weight loss has always stalled when my net intake wasn't high enough, which sounds crazy, but it's true! also, hey, it's only one day, see where you are in a week. it might just be a normal fluctuation