Why am I still coughing after I quit smoking weed..?

Your lungs can't repair the damage overnight. It may take up to a year for them to return to their normal capacity. Oh, by the way, it's Phlegm, not flem. I highly doubt your Doctor is stupid either. She/he has only had at least 5 years of post secondary education. My advice to you, is to stay off the drugs, and continue your education.