I've had both hips replaced and had so much pain in my knees I was afraid they were going south too. I asked my doctor about it and he told me that it's very normal, in that situation, for the pain to go down into the knees--I forget all the details but it's normal.
As soon as I had each hip replaced, the pain in my knees went away, which was a good thing because for a while, my knees were hurting almost as much as my hips.