hi troyrobins, if you are a smoker then most common cause is chronic broncitis caused due to smoking
and it can in future also cause cancer. So if you are a smoker quit smoking . Other cause could be many to name a few common ones, asthama, hay fever
, gastro-eosophageal reflux etc. For the current situation you need to get a fresh x-ray taken as the old one will not tell the present state of your lungs. The brownish fluid is not a good sign and so you should visit a chest physician to rule out any sort of pneumonia
or other severe lung infection
or disease. Do not neglect this condition as detection and treatment at the earliest can save lots of morbidity later. Take care.