When you quit smoking, your body goes into stress mode, as it is craving nicotine like none other. This leads to stomach cramping, stress, headaches, shakiness, irritability, and even nausea, as your body tries to deal with the brain waves that are just screaming for the nicotine its addicted to without getting the craving that it wants. Believe me, I know. I have quit smoking twice, and each time it was torture. I wish I knew about bananas when I quit smoking as it would have likely been a lot easier to deal with.