What Causes The Accumulation Of Yellow Fluid In Lungs?

I have pleural effusion but do not have cancer. I have RH in my entire body and would like to know what actually causes the fluid to keep building up and having trouble breathing. I have a drain tube and have to drain once a week, sometimes more but the doctor is unable to find the cause. Sometime the fluid is yellow and the doctor when we first started draining said it had the RH factor in the fluid but sometimes it is a strawberry color. Is there anything you can help me with.