I would personally have her checked for pain first if you haven't already. Because can you imagine how bad you will feel if you get further down the track and find out she actually was in pain and you were walloping her for trying to show you? And being walloped for being in pain isn't going to do any good, assuming there's a possibility of pain. (Been there, done that, got the T-shirt)
My gelding gets really cranky with the girth and at first I just thought it was him being a jerk, not wanting to be ridden because he was lazy and a toad etc.
Turns out he had a locked shoulder and his wither was out, so in actual fact he was hurting, but I took it as attitude to start with.
I would also agree with the others about checking for ulcers. We know our horses can't turn around and say hey I've got a sore guts, like we can to our friends and family, they have to make their pain known somehow and we know that. She may have been in pain for a long time now, assuming that she hasn't been checked over by a licensed vet/chiro/saddle fitter already.
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms