Question about bareback riding?

Today when I rode my pony bareback I noticed something. My mare has a wonderful smooth trot, that takes little to no effort to post or sit. It's almost like natural posting. It's great. But when I trot bareback, it seems like her spine curves on the step where I go down when posting. I'm afraid I'm hurting her.. Under saddle her ears are always up and alert but bareback they kind of droop back. Not pinned, but midway between up and back. Could I be hurting her? It kind of feels like her spine moves and bounces with me.