Hollypooh,
Knee discomfort after a total hip arthroplasty is fairly common. It can be caused by several different things.
One of the most common causes is the same as what was going on in the hip joint, osteoarthritis. In some patients, they have OA in many joints, but the pain in the hip was significant enough to overshadow what was going on in the other joints. Then after the replacement, the pain in the hip is decreased enough, that the pain in the other joints starts to show up.
In some patients, after the THR is done, the limb is slightly longer than before. This can change to gait mechanics. Thus the patient may develop knee or foot discomfort from the change in the way they are walking.
Some patients have referred pain from the groin region to the medial side of the knee. This is due to the way the peripheral nerves work in the lower extremity.
Sort of associated with the first reason given, the patient can have other problems going on in the knee other, than OA. It is possible that the patient has a torn meniscus or ligament instability. Again, once the pain from the hip is decreased, then these start to show up.
If the knee pain does not lessen over the next few weeks, have your surgeon evaluate the knee. Make sure that there is not something else going on in the knee that needs to be addressed.
Good luck.