What Causes Swelling And Pain In Knee Post Surgery?

Hi. I have problem with my right knee and hip. Now my knee is swollen and hurt me a lot. Two months ago I have my lower back surgery, because me and my doctor thought the back causing my leg pain. Right now I am taking Hydrocodone and Fentanyl Patches , those help me with my back pain but not my right hip and knee. Arthritis? Thank You for Yours time. Sincerely; Ewa Bor