Sorry but you're not going to be building muscle after just one week, but it may be down to water retention. If you are going to be exercising you will need to up the amount of water you drink - i've been told an extra glass for each half of exercise. Don't feel down and this is a temporary thing and the scales will start moving again, and exercising can only help.
Oh and you should definitely be having more syns - minimum of 5 a day but aim for 10-15 and see how you get on.
Good luck!