Hey don't worry about it - the beauty of SW is you can draw a line under yesterday, and start again today! Just as long as you don't carry on pigging out, you've still got a good chance of a loss - a whole week's good work shouldnt come undone over a small blip!
Get a bit of exercise in there and behave before WI and who knows, you might get a good loss!
Don't beat yourself up... that's what I always used to do, have a blip and end up giving up... with this plan, you really can do it for life, so dust yourself off and try again! xx