What Causes Random Pain And Swelling In The Joints?

My wife has had random pain and swelling of different joints ( one time in the left shoulder, which has gone away) and now is in the left knee, right wrist and knuckle of the right hand. She has had x-rays, because she fell face forward on her hands and knees a month ago, with nothing appearing on said x-rays. These symptoms have come and gone over the past two weeks, moving from either the wrist or the knee or the hand. Currently, her left knee, right wrist and the right index finger knuckle are swollen and painful.