If you have a sandy paddock or if her teeth are bad and she drops a lot of grain and then eats it off the ground she could ingest sand which could cause irritation in the intestines and even colic. An easy way to check for it is to take a bucket and fill it with water, then drop some of her poop in it, let it sit for 10 or 20 minutes and then drain the water, if there is sand, it will be in the bottom of the bucket.
My older horse has soft poop always, it never go's normal anymore. It's like a cow patty now. I have fed him yogurt before to replenish the bacteria in his belly and I had him on a full source feed that made a huge difference for a while but he only has 4 teeth now so he is on this other feed that is almost a powder now.
It could be so many things. Have you checked her temp?