OMG how devastating babes...could it be because you are "starving" yourself? If you eat below 800cals per day your body goes into "starvation mode"...? I don't know, I'm no expert but maybe try and up your calorie intake (I know that sounds stupid when we're talking "diets" lol!) and make sure each meal/snack is every few hours to keep your metabolism up.
Exercise can be key in a lot of diets too as it increases your metabolism and fat burn..might be worth a try if you're not already doing a little bit daily/every other day.
The only other thought would be your water intake..maybe try and up it a little bit to see if that helps, but not excessively of course as that can do damage.
Are you due on soon? Maybe having too much salt and therefore retaining fluid? Just some food for thought (maybe not the right phrase!)........chin up hun, you may get a wonderful surprise next week and lose big to make up for this week.
Take care babes xxx