What Causes Coughing And Running Nose At Night?

my daughter 15yrs coughs at night when she goes to bed for approx 45mins and blows her nose,when she is asleep shes ok,then in the morning the coughing and blowing of the nose starts again.During the day she is fine.We have tried benadryl plus along with the full range of antihistamines available over the counter to no great relief,she had suffered in the past with allergy symptoms late spring which went early summer,the symptoms she has now started aug 2010 and have gradually got worse.