dont worry, it's only a blip, try and look through some of the other posters diaries, i've noticed on a few signatures that some people have reported losses on their weekly weigh in even when they've had 2 or 3 days off! there was one diary (sorry can't remember whose it was!) where she had 2 days off one week and lost 1lb and the other time she had 5 days off and only put on 2lbs so really it's not all that bad! she rectified it the week later i think by doing ss and losing 6lbs so as long as you get back on track you'll be fine!
if i'm totally honest i've planned to have a day off the diet when i complete 6 weeks of ss because i think i'll need that little break to stay focused and just be able to have what i want for one day, even if it means its harder to stick to ss the following week!