Hi funcy, the blood can be irritation, infection, or part of the cancer. Lymphomic cancer is cancer in the lymph system, lymphoma. There are lymph nodes in every part of the body even in the lungs. Has she had any swelling? How often is the fluid getting in her lungs? Some people actually develop lymphedema and the fluid settles in the lungs as well.
Sorry to hear about your Mum's issues.
Is it possible that the fluid was not IN the lung, but around it, caught in the pleurum, which are sacs surrounding each lung. Any tumour in the lymph nodes in that region can cause them to cease draining properly and cause fluid build up.