Hi Melina,
On & off, for the first 3 months - maybe more - I thought I had a different an separate problem that related to my lower back and not my hip - or - it could have been related to my hip - I was never sure.
The point is that I had been getting back pain (that feklt like sciatica) which I've felt before. Other than having some stenosis in my back, they never really figured out where the pain (or ache) was coming from. But every pain I've gotten since my hip replacement - I related it to that surgery, or it was my body trying to adapt to my new hip.
Then suddenly the ache would go away, just as quickly as it came. This has occurred so many times in the course of my healing process, that I don't worry about it much. It comes, it goes, and I really don't know why - but mostly feel fine.
I can remember me at 9 weeks post-op too! I remember having the exact same concerns as you do now. My lower back was hurting me for awhile, and I didn't know why. This happened on & off. As always, it went away on its own, and my concern was for nothing so much of the time.
I really think it takes awhile before you entire body completely adjusts to a new hip! That's the way I look at it now. Or try to. If you need peace of mind, ask your Dr. to do an x-ray of your back to see what may be going on there - or an MRI.
Trudi