Has she always done this or is it something new that she is doing. It could be a number of things, from your cinch style, cinch size, saddle fit, to something more serious as back soreness problems, wither or rib isssues (ie. broken or fractured, or bruised). Before I would call out a chiropractor I would schedule an appointment with your vet and have a good exam done. That way if nothing serious is found you could try an acupucturist (great for back sore horses) or a chiropractor. Also make sure that you have you saddle properly fitted (not using the pad). You would be amazed what a horse will do to keep an ill fitting saddle off their backs.

Hope this helps and let us know what you find out.