What Causes Pain Behind Knee And Fluid Retention Near Knee?

My leg is hurting behind the knee and looks like I have fluid around the front of my leg under my knee down the front of my leg,I have had this pain in my leg since yesterday after driving about 40 miles went to get out of car and couldn t stand up the back of my right knee was in very bad pain and still hurts now after 24 hours I do need a knee replacement on right knee already had left knee replacement