According to Mrs Chang from Sensible Health, when you push stones during the flush, we tend to eliminate the stones which are in front of our gb. Then the ones from the back (older ones) move forward. This tends to cause pain in the gb meridien which crosses the upper back.
I, however, notice that when my body is asking for some cleanse I start having upper back pain. Then I flush and pain subsides. If pain persists I just wait another week and flush again(in this case I do not wait 2 weeks). After the second consecutive flush my pain vanishes for some months.
Hope this helps.