My mare did this frequently too and when her mouth got near mine.... well.... let me just say that me and her had a scratching, biting, all mighty fight that left both of us with patches of fur and skin bruised and looking disheveled. And you know what? She still loved me after that. Sometimes that's what's needed as bad as that sounds. I mean heck - she's nearing her twenties right now and we STILL fight like that occasionally, but we still go to great lengths to do things for each other.
If that doesn't work what I started doing was put my hand in between the girth and her belly as I was tightening it (removing my hand when I had the loops started so my hand wouldn't get caught) and that helped unruffle her fur, a felt girth also helped her. Now she just pins her ears and looks sour but a stern "Don't you dare!" and she straightens up. She hasn't tried to bite me since that fight. As for the puffing up give it time, girth up in stages. Where I worked we had a horse that did this and after trying to be nice about it my boss gave him one quick smack to the belly and he never did it again (he was doing it to be a brat).
All of this is assuming that there is no health issue. But trust me, I've come to learn with horses our version of smacking, walloping is equivalent to the beating they get in the field on a daily basis. (excluding abuse and definitely not condoning it!)