I agree that she may have stomach ulcers, or she could also have some other type of digestive issue that should be addressed that is causing her to not completely digest her food like she should, though there should normally be some hay in her poop. I would have the vet do some blood work on her if you can afford it, and see whats going on. I hope you figure it out, and she starts to gain weight again. But also keep in mind that some horses have a naturally skinnier body type, though generally that's more in Thoroughbreds, and racing quarter horses. I had a Thoroughbred who was just naturally on the skinny side, and you could see, a couple of her ribs, and could feel all of them pretty easily, and the vet said that that is just her normal build, and not to worry about it, but just work on keeping her at that weight, which is what I did, even though I got complaints from other boarders that she was too skinny. Just have your vet look at her, and see what he/she says.