What Causes Cough And Phlegm In A Child After Feeding Formula Milk?

Hi doctor, From day1 at hospital am feeding my son wit lactogen. Since my milk flow was very less. I then shifted to nan pro 1 from 3rd week since his motion was hard and now wit Nan pro he does not have constipation or motion proble and he goes motion once every day. I breast fed few drops a day till his 1month and then exclusively fed formula. His birth weight was 4.1 and now heis 3 months old and is 7.1kg now. But he is geting cough n phlegm each month.. isit bcoz I am formula feeding. Pls advice on this will my son have anynprpblembeing formula fed