Could be allergies or some mild infection. If she feels well and does not wake through the night with a cough, you could try an over the counter cough med. If she feels bad, go to her doctor. Should have a regular doctor that knows her.

Make sure you do not have candles, potpourri, etc., that may be respiratory irritants. She may need to be checked for bronchitis or sinus drainage that is keeping the cough going.