OK firstly, don't worry.
It might not be the case that you have put that much weight on. Are you using the same scales? Have you moved the scales? Are the scales breaking? Are you wearing the same clothes? Might you have retained some water?
I once read that I'd put on 2lb and an hour later my scales said I had lost 3lb! They fluctuate, particularly the ones we have at home.
Stick to the plan. If you are doing SW with a class, wait until the weigh in at your class.
If you are going it alone, consider setting a same-time-each-week appointment with yourself at Boots or somewhere with professional scales.
And, if you can (I know it's hard!), try and weigh yourself weekly. Don't become a slave to your scales!
If you have actually put on, think about what you might have done wrong. It might be something little (weighing your healthy extras, using the wrong type of milk, thinking something is free that isn't).
But DON'T give up!
Wait until Monday and see what happens on the scales then.
Good luck with it all and don't worry,
Ste
x