What Causes Pain And Swelling Behind Knee While Having Arthritis And Osteoporosis?

I have had pain and swelling behind the left knee for almost a month. I have arthritis and osteoporosis. Knee had been fine before. It is warm to the touch, I sweat a lot in middle of the night. Over activity worsens the pain. Have iced it on and off but nothing seems to take it away completely. Before pain started, I was walking about a mile a day, 4 times a week.