IF this is pain related, and that is big if, but needs to be checked out, someone, even a vet merely looking at horse is not going to tell you what is going on, unless x-rays and more intensive diagnostic tests are done.
For instance? Broken ribs, broken withers process, broken vertebrae, peritonitis, deep seated abscesses in muscles, internal problems, (saw gelding one time that had testicle lying right under spinal column for instance) mare could have ovary problems similar to that.
Teeth abscessed, broken jaw, broken bars, broken teeth?
All of these things are not going to be easily notice, by merely a cursory inspection.
Behavior problems are whole other ball of wax. This mare could be doing all of this because has learned she can, and get out of being ridden.
If you are using same saddle each time? Could be something wrong with it too, and not just too narrow, but underneath the leather. I've seen saddles that had nails/screws/wood pieces work loose, or gotten in there, and were poking horse when rider added weight to saddle. Turn saddle upside down and press with all your weight on underside of saddle, moving hands slowly while you do so, to make sure nothing is wrong with inside of saddle. Do the same with saddle blanket. Have seen sewing needle tip come out of one blanket years ago, must have broke off from machine when stitching, but there it was. Had forgotten all about that until just now.
What breed is this horse, she sounds an awful lot like the Tri Jet mares/fillies, that are TB. Real heifers to deal with.
More tests are in order here.