Go to the emergency room. They will probably take a film. You may have a long wait, but the procedure itself takes a very short time. There is most likely fluid build-up that can be easily removed with a hypodermic needle. Usually ice or heat won't help. The procedure is not painful, but the needle looks very big so look away and close your eyes when they do this. Sometimes this happens spontaneously without any special injury that the person can remember. They will look at the fluid taken out for evidence of blood and evaluate you for a possible torn miniscus. Often when the fluid is removed the knee returns to size and the problem simply disappears.