Life is never going to let you be 'good' all the time- sometimes things happen which you either can't control, or actually just want to enjoy yourself rather than worry 'oh no, how many syns in...' etc etc. Life doesn't work around your diet, you have to work your diet around life- and for most people that inevitably means having some 'off plan' moments!
For example, I used to LOVE Dominoes pizza and me and my OH used to get it regularly- I've cut it out now, but I know that one day, I will eat pizza again and enjoy it- the difference is that I don't do it every week. SW is about flexibility, and in this case you were flexible. Don't feel bad, accept that you may have caused yourself to STS, and move on.
As has been said already- it might change by tomorrow- if you WI at group, and you were using different scales, that will make a difference, as will time of day, clothes worn.
See how you get on tomorrow- might not be as bad as you expect. Good luck
