It is the nicotine
contained in cigarettes which is an appetite suppressant
Following are the reasons why a person gains weight on quitting smoking
1....smoking can increase the body's metabolism
(smokers' bodies have to work to remove the toxins from cigarettes)
2....smoking affects how you taste and smell food (people often rediscover the joy of food when they give up smoking)
3...some people put food into their mouths rather than cigarettes,as they have the habit of putting some or other thing in he mouth.
Just dont worry about it,as this shall settle soon with calorie reduction and exercises.
The benefits of quitting smoking are enormous and so just don't start smoking again.