What Causes Headache And Shallow Breathing After Quitting Smoking?

Hi,

I just reached my two week mark for not smoking. I smoked about a half a pack a day for the last 12 years. The first few days after quitting I felt great. I could breath better and I didn't get winded when exercising. Then around a week and a half into it I started experiencing shallow breathing, headaches, nausea, and fatigue. Is that normal? How long will these symptoms last? What can I do to help reduce how terrible I feel?