My Daughter Has A Continuous Cough, Unrelenting For Close To

my daughter has a continuous cough, unrelenting for close to 1 month. She does not have any other symptoms. Tests revealed that she has no asthma. The cough is dry,and comes after a cold. It happens every year twice a year, but this time is lasting so long. She is weak,horse and unable to study. Can you help me in this regard? Thanks very much.