There are lots of benign, typical things it could be. Like your doctor said, it might not even be something on your lung. Could have been a nipple. There really is no use speculating! We aren't doctors, you aren't a doctor, and speculating only gets our minds wandering. It is whatever it is, and once you get your CT, you'll know! It's totally normal to be anxious in such a situation, even if you didn't have anxiety issues to begin with. Being nervous about it is natural. That little buzz in the back of your head would be there for anyone - you just have to try to ignore it and not focus on it as much as you can.
As far as your smoking history goes, smoking history with lung stuff is really hard to pinpoint. There really is no way to say that one way or the other it did or didn't affect you at all without the proper tests. Some people smoke for decades and have great lungs still. Others aren't so fortunate. Statistically, though, the odds are in your favor. BUT, like said before, if it's just a nipple or a shadow, none of the speculation even matters!