When one has swelling of the knee, one needs to consider these questions:
Was there a recent knee injury?
If so, there could be blood in the knee cavity due to damage to internal structures, such as a torn cruciate ligament or a torn meniscus. The bleeding could be tense and painful, with the knee blowing up like a balloon due to blood filling the knee capsule. This is called an 'haemarthrosis'. A tense painful haemarthrosis may need the blood to be aspirated and the knee washed out.
Bleeding may also be into more discrete areas like the lubricating pockets or bursae which exist around the knee. Bleeding into a bursa can cause a painful and tense but discrete swelling. It can look more alarming that it really is, as there are no important anatomical structures in the bursae, but they may still need attention from a doctor, and may need aspiration and washing out to prevent adhesions forming and to allow the knee to move freely.
or perhaps a knee injury a little time ago?
Old blood in the knee may be causing irritation of the joint capsule lining, and secretion of excessive lubricating (synovial) fluid into the knee cavity. The fluid is likely to be dark brown and is called a 'post-haemorrhagic effusion'. This is not an emergency but may need attention from the physiotherapist.
Or did the knee swelling seem to just come on slowly without a knee injury?
There may be an irritation within the knee cavity, possibly from an arthritic process inside the knee joint. The fluid causing the swelling is unlikely to be blood but could be straw-coloured and watery. This is called an 'effusion'. The doctor may want to do some investigations, such as an X-ray, to see what might be the underlying problem.
Or did the knee swelling come on by itself, but suddenly and with the knee tense and hot?
There may be infection or inflammatory disease (like gout) inside the knee. This needs the urgent attention of the doctor, who may want to do blood tests and an aspiration of the fluid for culture and examination.