relax.. take a day or two off the plan and don't weigh or measure anything, have a few extra syns that day too..
it could be down to either stress or metabolic reasons..
both of which can be "cured" by the above..
you chill out and stop producing stress hormones which make you retain / gain weight, or you give your metabolism a bit of reasurance that you're not starving by giving it a relatively high calorie day..
If that doesn't work then there's also the posiblities of..
A. you're too complacent with the plan and think you know it off by heart now but you're having extra syns that you don't count or consider syns?
B. You've fallen into the trap mindset of "I lost X lbs by eating this last week, if I eat LESS this week I'll lose more surely? ".. which isn't necesarily true if you cut down on superfree / speed foods