it sounds as though she is sour. Have you taken her on a trail ride to get her to relax adn be happy ? What are you schooling her in ? Is it causing her pain ? Example jumping fast turns ? Hard arena landing ? Going into a fence crooked not setting up correct ? Or if , reining, sliding , stopping turning fast could all cause pain and grumpiness . You may say you are soft on the bit, but do you have to much pressure, to little pressure ? Bit setting in her mouth correct ?
Saddle pad not clean , something poking her? Same for the cinch/girth ..