The nicotine is what CAUSES the stress in the first place, and you have to relive it by smoking or ingesting the nicotine in another way. So essentially what you have done is raised the baseline stress level and you temporarily lower it when you get your nicotine. Then the stress levels will be ascending proportionally to the nicotine concentration in your blood which is constantly metabolized and descending. Nicotine has a half life of about 1-2 hours, so it doesn't last very long.