Sometimes when i smoke i get numb and lightheaded. why does this happen?

Few possibilities: It could be from a nicotine rush, blood pressure and heart changes, or a psychological craving reaction. I doubt one cigarette is dropping your oxygen level enough to do this however smoking is something that you should stop doing as you can already tell it is affecting your body and you don't know what it is doing to your lungs, heart, and brain until it is too late.