What Causes Pain And Swelling In The Left Knee?

I had bi-lateral TKAs in 2007. Both knees have been doing great until this past Saturday. (I walk between 7.5-10 miles per day) at approx 3:00 my left knee started swelling and painful I d walked about 4.5 by then but had done nothing unusual by 11:30 pm the knee was so swollen and painful I could hardly stand it. By the time I returned home on Sunday night it had gotten progressively worse. I went to Ortho urgent care this a.m. Where they withdrew 40cc of bloody synovial fluid for testing as well as various blood tests. The bloody synovial fluid was the exact color of what was in the CBC vial! What would cause that much blood for a non-injured knee. Am waiting results but am rather anxious about that blood red synovial fluid