It seems that the precise cause of your pleural effusion (water in the pleural cavity) has not been determined. I am not sure if they took a little strip of your pleura (biopsy) when they took the fluid, as that might have produced the answer even if the fluid did not. TB cannot be excluded simply if the TB germ was not found in the fluid, which may have been sent for special culture and the answer takes between 2 and 3 months. In short the process that caused the accumulation of fluid may still be active giving you the pain. I would suggest that you should go back to the doctors who investigated you for an up-to-date check up.