I think it is somehow related to expected weight re-distribution and the lower centre of gravity as a consequence and necessity to carry and display our bewbs proudly

. I used to have some issues down there and I did a lot of stretching and yoga exercises, but after things got "re-adjusted" by the HRT, I am not getting any lower back pains, my flexibiity has increased and I enjoy walking with my back straight (as opposed to hunching like an ape).
One thing however - which can be good or bad, depending on circumstances

. I did some effort with practicing the female gait and one of the preconditions was straightening Your back and pushing Your shoulders a bit back - at that point Your butt will move forwards automatically....
So after my back had readjusted, I sometimes realise that this gait has kicked on automatically - because it is very comfortable now to move from the hip instead of the knee - and I need to tone it down a bit

.