I Am 78 And Have Had A Knee Replacement About

I am 78 and have had a knee replacement about 8 years ago. Four months ago, my "new knee" became very sore, with lots of pain on the outside & and slightly below the knee cap, making walking upstairs- very painfull.My Arthropod took a lot of fluid out at that time, but no relife. MRI shows the replacement looks fine and the doctor feels that the replacment is in good shape. I started P/T, for the knee, but after 4 weeks - no change. My leg also aches at night when trying to sleep or if sitting in one position for a long time.

Any suggestions?