I had the same problem with my western saddle last year and took the
horse and equipment to a trainer who told me I'd been putting the saddle
too far forward and then holding it there with a breastplate. The whole
saddle tipped back, though it wasn't really noticeable until she pointed
out that the seat wasn't level as we stood and looked at it. With the
saddle tipped back, my weight was being distributed more heavily across
the loins. She said this is a very common problem.
My Sportssaddle naturally slides back to the same spot. My horse also
moves me back there as we trot and canter bareback, so I guess I've been
wrong about where I've wanted to sit all these years! :-) ~Nikki