Suggest Treatment For Pain In Knee

I injured my femoral tendon (right knee two years ago) since I have not been able to excercise as before. Everytime I do my knees (below the knee cap) are in pain. Recently I was at a wedding and danced; since I've had pain in my left knee. Today there appeared a red blotch on the top of my knee. How serious is that? They xrayed my knee-no mri.