I too think it's just your lungs clearing out the garbage that was left in there, which would mean it's a good thing, I think. If it doesn't go away, then by all means, see your Dr. I didn't cough up anything, or ever cough AFTER I quit, but, I used to cough up that same brown thick phlegm WHILE I was smoking. I quit 2 and 1/2 years ago, and I wish the very best of luck to all of you new quitters!!!
Stick with it...it is SO worth it!!