Two nights ago my knee suddenly swelled up and became painful. It is still swollen and painful, although I can walk easily and hardly feel it during the day. In the late afternoon and night it is more painful, but mainly only when walking, moving. There is a reddy/brown patch showing below the knee, and the knee itself is somewhat red. It s painful to touch, although again not unbearably, and it feels like there s maybe fluid in the swelling. what do you think it might be?