Your weight is not a static number. It can fluctuate several pounds in a day and week to week! As fluid shifts (as you eat, and especially around your period) you can "gain" and "lose" several lbs... that doesn't mean fat.
If you've started exercising, it's not even unusual to gain weight a bit at first! More info here: www.sparkpeople.com/community/ask_the_expe
It can take as much as 6-8 weeks for lifestyle changes you make to show up on the scale!