Congested Heart Failure?

My oxygen drops to 84 at night while still awake, but 94 during the day. A CT scan shows small Pleural effusion in my lungs. CT scan also showed small Pleural effusion four years ago. Echocardiogram was done and my heart looks healthy. I do have some slight chest pains at night on and off. For the last 9 months I sleep up to 10-12 hours a day. Prior I would sleep about 7 hours. The lung doctor thinks it is congestive heart failure and says a echocardiogram may look normal and you can still have congested heart failure. But, my regular doctor thinks because my echocardiogram was good and my blood work is good that congestive heart failure is not the problem. She thinks the pleural effusion was from me forgetting to take my thyroid medication and because my thyroid tested low. But my thyroid was normal four years ago and a CT showed Pleural effusion then too and it still leaves the question why I have a drop in oxygen while laying down still awake. The lung doctor says sleep apnea happens only when your asleep and my overnight test shows it drops while awake. Any reason for concern for heart disease?