Stop smoking, it is a stimulate, nuff said... yes, I quit 6 years ago...
Once you are addicted to nicotine one perceives that it calms them, when in reality they are getting their "fix" of nocotine which is addicitve. Not having something you're addicted to will cause anxiety, and when you get what you want of course you're calmer. I wouldn't expect this to be the same with a casual smoker.
No. But it is a possibility. Check effects of nicotine on brain.