Hello and hope you are doing well.
A positive methacholine challenge test would indicate there is some amount of bronchospasm. Your symptoms could be due to small airway disease, which affects the respiratory bronchioles. Disease of small airways results from exposure to tobacco smoke, mineral dust, air-pollutants, and viral infections. It may also be associated with connective tissue disorders. The pulmonary function tests are abnormal only late in the disease and the damage done is irreversible. Therefore, management lies in preventing exposure to smoke and dust, whether in the form of smoking tobacco or exposure to industrial dusts in the work environment. Please discuss this with your doctor am sure he will provide further assistance.
Hope this helped and do keep us posted.