I would go to a vet. They may want a placenta sample. It could be, not a big deal, but it could be some thing like bvd and you may not want to leave her with the herd.
Some humans can't carry a pregnancy either. It happens. I've invested money in placenta and fetal tests and such in the past searching for answers and only poorer for it. If she's the only animal doing this chalk it up to the mystery of life and death and move on.