Suggest Treatment For Pain In Lower Legs And Swelling On Knee

I was walking with sandals on 2 days ago, then my lower leg start to have pain & then my knee starts to swell & have pain. My knee has been in pain for 3 days now. I have to raise my knee to keep it elevated & iced as my knee hurts & swells. I am going to the Dr. tomorrow but wanted to know if you knew what was wrong?