All the crap that's been accumulating in your lungs from smoking is coming out. My dad had the same thing when he quit- he felt worse at first because of the cough. I got a cold or something when I quit which isn't gone and I think all my lung stuff is starting to come up too. My upper back was a little sore yesterday and is again today, but I'm not 100% sure if it's still the cold or not that's giving me the cough. Try to stick with quitting, I'll do the same!