You need to post some pictures of her saddles up, pictures with each pad and pictures without any pad. The saddle could be too long for her, the back skirts may be too straight and digging in to her loin, or the pad could be too long and digging in to her loin.
For horses with tender backs, I like to use a 1/2" real wool felt pad (PLAIN) that is contoured to the back, along with a ThinLine saddle pad
on top. OR a Skito saddle pad
with quilted felt bottom and 3/4" foam inside. Both will give good cushion/shock absorption without causing any friction or any possibility of digging in to the loins.
I would have a good equine chiropractor out to rule out a physical problem that may be causing the back pain. The way she travels in her rear could also cause loin tenderness. You might have someone else ride her and watch her gaits. If she's stepping short with her rear legs, she could have an issue in her hip, pelvis, or with the shape of her feet.