What Causes The Formation Of Yellow Fluid In Knee?

My daughter injured her knee in an accident five yrs ago; went through physical therapy and was doing fine. She reinjured her knee when she whacked it on the corner on a table which she was lifting and moving. MRI shows no injury, no meniscus damage, no joint damage; but, her knee swelled up at the top and became very hard a few weeks ago. She finally saw someone today who drained it. The fluid was a yellow color. He was sending her to a rheumotologist, but she can t get appt. until March 23rd. Meanwhile, she called to tell me that the knee has filled up again. What should I tell her to do? What could this be?