What to do about a horse that is "cinchy"?

You might want to have a chiropractor look at her. My mare does this when she has a rib out, but a quick adjustment fixes her. She might also be remembering pain from a previous rib out or injury and she just reacts that way because she expects pain. When you tighten her cinch, put it tight enough so the saddle won't come off. Then do something else, like pick feet or brush mane and tail. Then tighten it a little more and do something else. And so on until it is tight.