Everyone worries about their weigh in, how it will go, especially the first week, when you cant quite believe it is possible to lose weight and eat all that stuff. Try not to worry! If you follow the plan accurately, and stick to it properly, there is no way in the world that you would gain body fat AT ALL. If you are worried whether you are getting it right, start a food diary in the food diary section and post the stuff you have been eating the last couple of days so we can check it out for you an iron out any minor issues or things you may have overlooked.
Lots of things can cause temporary gains, none of which are "because you are following Slimming World properly". Are you doing this with a group or at home? Even just using two different sets of scales can make a big difference. Good luck!