I think
smoking cessation and weight gain sort of "automatically" go together, just like hypothyroidism and weight gain go together. I quit smoking about 6 years ago and was diagnosed with hypo a year later, though I'm pretty sure I was hypo for a good while before being diagnosed.
You may be cutting your calories by too much. You need a certain amount of calories, just to live - to maintain bodily functions: heart, lung, brain, liver, kidney function, etc.