About 7 months ago I had a collapsed left lung with chest tube and surgery. Burst bleb. Every time I cough or sneeze I feel pain in left lung. Why?

Adhesions. Scar tissue. The lung gets stuck against the rib cage and the ribs are not fully healed yet, either. You need a good session with the DR who treated your pneumothorax. Coughing much? Coughing blood? Short of breath? Rapid heart? etc.
Inflammation. Make sure you dont have a recurrent partial collapse. If not you may have inflammation of the pleural cavity or irritation of some of the nerves along your ribs on that side. You may need a course of anti inflammatory medications ir evaluation by a pain specialist as you shouldnt have pain this late. Discuss with your surgeon.
Mgt. There may be residual inflammation or an infection. It is important to see your PCP and to possibly have a chest radiograph to determine if there is an area of collapse.