Could be an infection. My first TKR was great for seven months. I had backpacked the Grand Canyon, and been downhill skiing, and then woke up one morning with symptoms that sound similar. It took a couple of months including very painful PT, but it was finally figured out that my knee was infected. Long story, but eventually my surgeon sent me to Mayo. It was a long ordeal involving removal of the original prosthetic, weeks of IV antibiotics, and eventually a new TKR. That was 10 years ago, and thankfully I’ve been backpacking and skiing again.