Hello Doc,I Am 20 Years Old.been Smoking For The Past

Hello doc,I am 20 years old.been smoking for the past 3 years,recently I have quit smoking since the past 20days,for these 20 days I have been feeling very weak,I have regularly been coughing and also getting cold. I had gone to a doctor where I got antibiotics due to which my cold and cough stopped but still I am having slight pain in the neck and continuously spitting flem,also I feel very tired and weak.