What Causes Knee Pain And Ankle Swelling?

I have this dull throbbing ache in my left knee. Earlier it was in both knees. I applied ice to both of them and took 2 Advil 5 hours ago. The left knee started again and might be what woke me up 2 hrs after I fell asleep. My left ankle is also slightly swollen. I have seen my doctor last week and after he examined my knees he believes that it is arthritis.