Some horse react that way when you tighten their girths too quickly. They think that they can't breathe. Some will drop to the ground, gasping for air, some will panic and run wild. It's very scary for them, they feel like they're suffocating. Try tightening the girth verrrryyyy slowly, one hole at a time. But it on at the first hole, walk her around in a large circle, put it up to the second hole, walk her in a large circle, and so on until it's tight enough. If this behavior continues, I'd consult a vet or an outside professional horseperson for an opinion. It's just not healthy for a horse to act this way. They'll continue to hurt themselves and potentially hurt other people or other horses. Chances are, it's NOT her fault that she reacted this way. It's important to not get upset with her and find out how to make her problems go away and make her a happy horse.