I am not trying to offend anyone but that is a huge myth
that minis and ponies don't need feed/grain and this is why you see so many that look like the horse above. Another myth is a handful of hay is all they need
I would get this girl on either Purina Equine Jr or Equine Sri asap. Both are complete feeds so you don't need to add anything and both are very digestible. Also feed hay. Remember to always feed up slowly so you don't cause colic.
Do you have any idea when she could have been bred?? If she would be past 120 days gestation I will mail you a free urine pregnancy test that you can do at home. Given her condition she probably didnt settle but you never know.
I have many years experience raising, breeding and showing shetland ponies and miniatures and I can guarantee you that every single one of them gets a good feed plus hay and pasture. And no none of mine are fat or foundered :)