Hi and thanks for the query,
this deserves a proper evaluation by her pulmonologist. A chest X ray although negative does not exclude disease. This could be due to a chronic obstructive pulmonary disease
, chronic lung infection
like pulmonary TB or a lung malignancy. It could be interesting to do a chest CT scan , cytology, lung fibroscopy could be important. Past history of smoking
is a contributing factor or predisposing factor for lung malignancy and chronic bronchitis
. A family history of asthma should also be excluded.
Consult a pulmologist for a proper evaluation.
Thanks and best regards,