Stopped smoking can't loose any weight

Hello everyone. My first time posting here. I am desperate. Stopped smoking 1stbof October obviously put about 12 lb.i have been using the app to log all my calories and also i am in the gym 5 times a week burning on average 450 calories. My food calories are set up for 1200 per a day.well after a month i haven't lost any weigh. Nothing. I worked so hard in the gym and also don't drink.
I have lost weight before the same way.3 years ago i lost 14kg for 3 monts and now nothing. Can't cope