It's difficult to tell without seeing your stroke (and it's difficult for me to explain in english, I hope you understand my wish-wash).
I'm not really competent, but I know three possibilities (there may be others too):
- Perhaps you "work" against this lifting of the body. It sounds queer, but I did that in the beginning. I never had pain in my back but after this exercise. When I "released" the body, letting this rotation in the short axis happen, it was easy to get air and my back wasn't sore after swimming.
- Or perhaps your arm sweep is wrong. Same as in freestyle, you grip and anchor your hand and forearm and pull your body forward (not pressing the hand and arm backwards). If you don't grip but try to push the water with your hands backwards it's like you try to wipe away any dust or something like that, but you don't get the force to lift the body.
- The timing: the face and shoulders go up with the insweep. If you try to get up with the outsweep, it's a bit like you try to work out of a hole. Your arms/ shoulder girdle and body are not "compact" you cant't use the core muscles to go up, you have to use the arm muscles which are much weaker.