Failure of the left heart to pump blood into the arteries results in 'back pressure' in the circulation which can cause fluid to accumulate in the air spaces of the lungs. This is most likely to occur when you exercise or when you are lying flat in bed and it may be accompanied by a cough. Breathlessness may actually interrupt your sleep and you may need to sit up or even go to the window for fresh air. But go to your doctor and get it checked out. Dont believe everything that you read, it could be anything, get a proper diagnosis