Pneumonitis is another term for pneumonia. If you had pneumonitis, it would have been seen on your chest x-ray.
Viral infections like a cold can cause inflammation of the airways of the lungs. This inflammation can cause coughing and chest tightness. After the cold is gone, it is possible for the inflammation to linger. This inflammation can last for several weeks. Sometimes this inflammation may linger for 3 to 6 months. Eventually the inflammation will go away, and then the coughing and chest tightness will stop. This is called reactive airways disease (RAD). When this inflammation is treated with an inhaled steroid, which is used to treat asthma, it often clears more quickly.
When this continues, it is generally considered to be asthma. To know for sure if your symptoms are due to asthma or RAD testing will need to be done. Generally testing starts with a simple breathing test called spirometry. This test provides detailed information about how your lungs are working. It will show if there is obstruction in your airways. To really test for asthma it is best to repeat this test after using a rescue inhaler, like the albuterol. This measures how much the bronchodilator helps your lungs by reversing the problem. When there is a 20% increase, the test is positive for asthma.
Of course, I have no idea if what you have is interstitial lung disease, but I would have a pulmonary function test anyway which can tell you a lot about your lungs. Please follow up with a pulmonologist, at any rate. I hope you feel better.
Dana