I recently developed a swollen, painful, stiff left knee, but have no history of trauma to the joint. Any ideas for the cause?

Time 4 medical eval. There are several possible causes of atraumatic knee swelling and pain. Inflammatory arthritis, degenerative arthritis, gout, and infection are a few of the possibilities. A visit to your physician should be able to give you an answer.
Get evaluated. There could be many causes including arthritis and infections. You need to get evaluated to get correct diagnosis and treatment options.