Hi Breathlessness of sudden onset while walking with no associated symptoms could be cause by an allergy or irritation of the respiratory passage by environmental pollutants or just stress.Other causes of breathlessness like lungs and heart problems are less likely here.The inability to breath was as a result of some spasmic movements in the respiratory tract.I will advise you just observe and reduce activity any time you feel it wanna happen again.If it happens again,you take an anti-histamine like aerus and see the Doctor for an evaluation of your respiratory system.In the meantime, stop smoking if you smoke, avoid exposure to environmental polutants, get into low weight diet and activities if you are overweight. Hope my answer will help you Best regards