There are a number of possibilities-
1) If you're talking about a short period of time, you starting working out can make you weight more in the short-term, as sore muscles retain water. You could also be retaining water for any number of other reasons (salt intake, time of month, etc).
2) You're not measuring/weighing your food or tracking everything so you're actually eating more than you think you are.
3) You're burning fewer calories than you think you are- the estimates in FS can be a little high.
4) You're building muscles and getting smaller, even though it's not showing up on the scale.
5) You're close enough to your goal weight that your body doesn't want to lose vanity pounds- limiting your deficits to under 500 cals may help if this is the problem.
6) You're unrealistically expecting to see a drop on the scale from day to day- that doesn't happen and you have to be patient and wait for results to reveal themselves.
I agree that you should pay more attention to what you're eating- I have a sweet tooth myself but you CAN curb it over time. Over the course of the last year, I've made huge strides in pulling it back. Make sure you're getting enough protein, which is especially important when you're working out. Be extra careful with your calorie counting, drink lots of water, and be patient- things don't happen overnight!