Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the node and pleural effusion that you experienced.
A nodular mass in the lungs may suggest several medical conditions such as tuberculosis, sarcoidosis, abscess, echinococcus cyst, tumor, etc. In diagnosis of tuberculosis three bacterial cultures from sputum should come back positive for tuberculosis. An abscess appears after lung infections such as bronchitis and pneumonia. Pleural effusion may appear as a result of inflammation of the pleura or when the pneumonia reaches the pleura and pus is released in the chest cavity. Transudate, which on an RTG may be also presented as a small collection of fluid in the basal areas of the chest cavity, may appear as a result of cardiac conditions such as chronic heart failure. Pleural effusions may contain pus (related to infections), blood (related to injuries of the blood vessels) or lymph as a result of injury of the thoracic duct. Any kind of collection of fluid in the thoracic or abdominal cavity should be examined with a previous thoracentesis.
Given the data that you described, you may or may not be experiencing any medical condition. Have an ultrasound exam of the thoracic cavity and if greater amounts of fluid are found, thoracentesis should be performed. You may also consider having the diagnostic test for tuberculosis to rule out this condition.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor for further advice and information about diagnosis and treatment options for your current condition.