I might try a different cinch. I’ve not yet met a horse who doesn’t like a fleece girth/cinch. Something like this http://www.ridingwarehouse.com/Toklat_CoolBack_Deluxe_Competition_Western_Cinch_/descpage-TDCC.html
It could be the metal buckle gets hot and feels like it is burning. It could be that the contact between buckle and skin feels too hard on her. It could be that the cinch is too tight. I’ve noticed that if you use a thick pad (like a 3/4" pad) there’s more of a pressure spot from the cinch than if you use a thinner pad.
I’d try a different girth in a different material - some horses really don’t like neoprene. It heats up and doesn’t breathe.
Think of how you feel if you wear pantyhose and when you take them off all you want to do is itch at your waistline!!