My biggest problem with Slimming world is weekly weigh ins.
I would never get on a set of scales until I was 4-8 weeks into a diet because I know my body, and I know the scales will demotivate me! You probably are losing water retention and fat, but the proteins will be strengthening your muscle fibres and if you add exercise to this, your muscle mass will increase = you put weight on.
It is extremely common, as woofgang said, to bang the water retention back on after coming off the Atkins,often coming off the Atkins depresses you more than you were to begin with!! It is a good crash diet for a holiday or occassion you want to look good for, but you WILL put weight on fast with the re introduction of carbs, even a small amount.
Give it time, I have been a personal trainer in my time and I know that often, especially with women, you reach a stubborm stage in weight loss that seems insummountable (for you the 12st barrier). Keep up with SW but dont worry about your first few weigh ins please!!