Chest X ray and sputum test can't detect lung cancer many times.
There are many causes of chest pain and lung cancer is not the major cause.
Go for HRCT thorax and if any lesion found ,it can be biopsied though CT guidance or bronchoscopic guidance depending on the location.
Next Steps
Give detailed history about the duration of the chest pain ,any other symptoms like cough ,blood in cough weight loss ,difficulty in breathing ,smoking history.