My horse is acting weird.?

When i put the saddle on my horse and tighten the girth even just a little bit she turns her head around with her ears back and looks at me funny. I thought she might just be getting fatter and its uncomfortable when i tighten it but if i leave her alone for a couple of minutes i can always get it at the same place. She started this 2 days ago and ive been using the same saddle, same girth and everything and i have always been putting it in the same place so i dont know whats up with her. Please help.