Since I have some experience in this from the clinical side, I can tell you that a lot of that weight is probably water that your body needed. Smoking dehydrates you (just look at the skin of a life-long smoker !).
There's also a sugar craving because tobacco is naturally bitter so the leaves are usually soaked in a 23% sugar solution which can give you that nice sugar/insulin response cycle (and partially explains why smokers have to light one up in fairly regular time intervals). The best thing to combat this is to eat some fruit.
Either way, congratulations on your progress and keep reminding yourself that you are now a non-smoker. One last tip: Any time you see someone smoking, think of a cigarette or, really, any time you remember to, take a really deep breath in. Many people don't take enough deep breaths and your body will sometime trick you into doing this by giving you the craving for a cigarette. It's not optimal, but your body considers it a bit of a tradeoff because it really requires those deep breaths.