With the sensitive, ear-pinny behavior when you touch her sides, plus the drop in weight and condition, plus the diarrhea, it definitely sounds like she has an unhappy gut.
My guy tends to go that way in winter when the pasture is gone because, like many, my boarding barn doesn’t provide what I consider enough hay. It’s fed AM and PM, and they clean it up in an hour. The rest of the day they’re basically picking at a completely dead/dormant pasture or just standing around with empty tummies.
Last year was my first year boarding after having my own place for twenty years, and my guy, who was always a super easy-keeper, lost a considerable amount of weight. He was about in the same condition as your girl minus the belly. No topline, ribs showing, hair coat rough (even for winter).
This winter I was ready and I’ve kept a trash can stocked with alfalfa/timothy pellets in my horse trailer. I feed him just under 4 lbs (dry weight) of them (soaked) daily. Even if I’m not going out to ride, I go out to feed him his hay pellets. He looks a ton better this year. Topline nice and filled out, no ribs, hair coat is great (that’s thanks to the small fortune in other supplements I’m feeding!).
I’ve also discovered that giving him to Probios soft chew treats a day keeps any winter loose poop away and just seem to keep his tummy feeling good in general.
It’s a little irritating having to supplement the hay when I’m paying $400/month already for pasture board, but oh well. By April the pasture will be up and there will be more than enough for them to eat 24/7.