Thanks for your query at DoctorSpring.com.
I will not be considering lung cancer in you straight away. However, I will suggest you to take a chest x-ray, spirometry, complete blood check now.
The first impression I get is probably asthma, however, you should have got relief with albuterol.
2nd thing I may consider is viral fever with muscle pain
3rd a small pneumonia
Rarely a pulmonary embolism.
I suggest you please mention the time you feel pain and shortness of breath, what increases it, any diurnal variation, any symptoms that occur during sleep.
Brown spots in sputum of an ex-smoker is usually nothing but the smoke inside.
Hope that helps.
Dr. Jacob George