What Causes A Burning Sensation Behind The Knee With History Of Hip Replacement Surgery?

I am 66 yrs. old. I have had a total hip replacement 5 yrs. ago. I have ceramic and Titanium because I m allergic to most metals. I can tell a difference when I have had too much acids. Today I have a burning sensation behind my right knee. It has been uncomfortable for some time now when I put pressure on it. For instance sitting in my recliner. I am a hairdresser and stand a lot. Why is my leg burning?