I can´t tell you for sure, but in my experience every time I flush I have some back pain, either upper back of mid-lower back pain.
It seems that as stones from the front of your gb moved out and the ones from the back are moving forward it somehow makes the meridian of gb more sensitive.
According to Mrs Chang the gb meridian goes through the back and as flushing means a big move to the gb it seems to be normal to have some pain.
Sometimes my pain is so intense that I have to take a pain killer, even though I know it is bad for the liver.
If have a lot of stones like I still do it seems to be part of the game to have pain. it should subside when we get rid of our damn stones.