Hi and thanks for the query, It would be important to know if there is any trauma experienced lately. Do you have any fever or other pains in other joints of the body? A clinical examination is necessary, especially when it comes to the knee, for its a very complex joint. If the knee is warm and painful, an inflammation is then suggested. Blood tests to quantify the degree of inflammation, a complete blood count to check white blood cell counts that could suggest a septic arthritis. Other tests like those for syphilis, in search for lyme antibodies in case of exposure to a tick infested area and an X ray to ascertain the integrity of the joint. Consulting your primary care physician is of utmost importance. Early and institution of an appropriate treatment will determine the outcome. Thanks and best regards, Luchuo, MD.