It could be that the infection is not completely gone, allergies/PND, dry air, particulates in the air,
acid reflux, a medicine you are on... It is really impossible to for us to say exactly.
It might be a good idea to see your doctor again to make sure the the infection is completely gone. You don't want that to come back as "super bugs" develop from incompletely treated bacterial infections. Certainly if the cough continues you need to see a dr.
Feel better.