We have had several people (including myself) who had issues with pain in several areas other than the hip after surgery.
The mechanics of the body have been contorted to help the body deal with the OA, when its effect is lifted you are left with a body that has some muscles and joints that have been overworked and some that may have atrophied.
Given the years that you faced it, it may take some time to undo the damage and relearn how to move correctly.
Having said that, I didn't have knee pain to the extent you're describing. The best thing to do probably would be to talk to your surgeon, describe the pain. Make it clear that your hip is not the direct source of the pain, and its manifestations, just like you've told us.
If he or she has you come in, the best thing is that it clears your mind and you can move on. If your knee is becoming irritated, then you can identify whether rest is the best thing for it. It is a marathon, so taking a long term view on this is valuable; you will get there, maybe not as fast as some, but speed is not the goal, full function is.
If you can't recuperate your hip properly because the knee pain is interfering, you are not accomplishing what you need to with the exercises. Get the knee straightened out until it becomes a help and not a hindrance to your recuperation.
Lastly - we've all had setbacks at different times. If you handle this to clear your mind and allow the knee to recoup, in the future you'll just look at this as a misstep.
Pain is a good guide that something is up, though, so talking frankly with your surgeon is key - they are here to provide service to you.