Same here. I have general inflammatory arthritis in most joints, hips, spine etc... and get a lot of back ache.
When I first went to physio she did some tests including getting me to lie right on the edge of the bed with one leg at a time over the side to see how long I could keep my leg held up. It wasn't very long and she said my hip muscles were weak and not supporting me properly which was unbalancing my body, putting extra strain etc... and causing a lot of pain as it's such a core area supporting your body (if that makes sense).
Can they give you any extra help to support your body better and get it more balanced to see if it helps the pain?