I always find that if I don't eat enough, I don't loose as much.... If at all!
I even some times have a habbit of skipping my 2nd shake (I have my 600cal dinner at lunch time and my 2nd shake in the evening) and when I miss it, I don't lose!
I've also tried having just 3 shakes instead of the meal and that really didn't work!
Make sure your dinner is only 600cals.... These things can sneak up on you.
Eat your snacks, there there for a reason, not just for when your hungry.
Drink plenty (2litres) of water or else your body will hold onto what I does get.... Making you gain water weight and bloat!
Like Roxan said is it time of the month or have you had a poop???
I find that when I exercise alot my weight loss does slow down on a day to day/week to week weigh ins but it helps so much more in the long run..... I wasn't noticing big losses so much and then it suddenly crept up on me
Keep at it..... Try using a calorie counter ( like myfitnesspal) to keep you on track
Good luck.xxx