2to 3 months , does the severity of cough increased ??or same ? Is  last Chest X-ray is normal ? Does the cough produce sputum  , If yes, is she on any antibiotics, Does she have any fever ? Go to a physician ,let him take a proper history and advise... Is she hypertensive, if yes .. Is she talking any tables ? Few of antihypertensives cause cough (ACE inhibitors ), ... Yeah it could be pet allergy.... After ruling out all the common conditions we can come to a conclusion...