My second guess is it is due to the Physical Therapy that I had yesterday. My muscles are sore and I struggle through my exercise routine this morning. I did virtually everything with a little assistance with the seated leg extensions. You would think something as simple as straightening your leg wouldn't be so difficult. I have found that the difficulty isn't that I can't do it. It is the thought of the difficulty and the pain that will cause while extending the leg. Once I initiate the leg extension then I have little problems. It is amazing the power of the mind; I can see how easily people don't go through the physical therapy or don't do there exercises. Once I put my mind aside and just do the exercises it is a little easier. Yes the pain sucks and it is not easy but it is much better than the alternative. The pain is not necessarily in the knee but the muscles and ligaments.
If you wish here is a short video on what the actually happens during a total knee replacement. It is worth the watch. Although this video may not be actually what I went through, but it does give you a pretty good idea of what happens. It will vary by type of knee and the doctor but those variations are minimal. My knee replacement was not cemented in place. He used some sort of cement-less alternative.