What Cases Pain And Swelling In Knee Without Injury?

My daughter is 13 and has swelling and pain in right knee. Has been seen by a doctor and he gave us a compound cream to be put on 3/4 times daily. We have been doing it for atleast 3 months now and at first there was an improvement, but now it has returned with a vengence. Already had a MRI showing alot of inflamation of the tissue. Was told exploratory surgery if this cream does not work. Dr. could not say what is causeing the pain and swelling. Do I need to get a second opinion? She already had surgery on right ankle do to cartiladge damage.