It could be a mild colic..
or maybe even a dehydration thing, if she seems listless and tired, she may be dehydrated or anemic or something similiar.
Make sure she has a lot of water, like keep an eye on that intake too.
If you have any kind of electrolyte thing available (they make solvents etc) id give her some.
Vitamins and electrolytes can really pick a horse up and steady them sometimes. If that helps her, try it, and keep an eye on her, just for a day or two, if no improvement or steadying, call that vet!