Don't worry! If you had only one or a few cigarettes for the first time of your life last week, there should be no permanent damage to your lungs or to your health. If you are concerned about detection by a nicotine (or cotinine) test done for insurance or employment purposes, even in regular smokers, any chemical traces are gone about 2 weeks after the last cigarette.
Never. The important thing is that you stopped. The damage to the elasticity of your lungs will never heal. There will be some selection against the cells whose genes you damaged but you'll always be at somewhat greater risk for lung cancer. The thickened, narrowed airways of the smoker heal readily when the habit is stopped -- and congratulations on stopping.