Hi, I Am A 58yom I Have A Sore Knee

Hi, I am a 58yom I have a sore knee for the past two and half weeks. Sudden onset, no physical injury. Knee initially felt "loose" with a tendency to over extend. Worsened over first few days. Been difficult to straighten it without pain. Condition has not gotten any worse or better for past two weeks. Been wearing a tensor knee brace which helps with walking. No previous medical issues with this knee other than occasional twisting or overextending while playing sports 20-40 yrs ago. Retired military helicopter pilot.