Your mare is definitely trying to tell you something is bothering her. Frequently girthiness is associated with ulcers, which are fairly common in horses in work. Treatment with GastroGard® (omeprazole) is highly effective, and following treatment there are several ways to help prevent ulcers from recurring. If you aren’t already familiar with treating and managing ulcers, there is much information on this topic on the web, and even the SmartPak Ask the Vet blog. Go to the blog page then you can type Ulcers in the blog search field and it will pull up several articles if you want to read further. By the way, scoping is great for diagnosing, but we actually skipped the scope and went ahead and treated our senior gelding with Gastrogard when ulcers were suspected, and the resolution of his symptoms following treatment was confirmation of the suspected diagnosis, and saved us the cost of the scope.
Western Pleasure, Hunter/Jumper, Working Cow...there's an App for that!