This post perfectly describes my situation. Out of HS 4 years ago, I started smoking a lot and it was great. I never had one issue, besides a lack of motivation. Fast forward to last fall, and my highs changed. I don't know why, or what caused it, but every time I smoked I couldn't sit still. Adjusting in my seat every 5 seconds. It was terrible. That led to me becoming paranoid about it, and my heart would race, and it would just take any fun out of the experience. I've been clean for a couple of months now, and I have no need to smoke again.
edit: I should also say that since I stopped smoking, I have experienced anxiety daily. I believe I am getting panic attacks, because my heart starts to race and I get light headed, so I'm seeing the doctor about this tomorrow.