They can get a reaction to bog asphodel, which often affects their ears.
If there is bog asphodel on the ground they’re grazing, can you get her onto drier ground where there isn’t any?
They also get a photosensitive reaction when anything has caused liver damage. She couldn’t have a fluke load affecting her liver function, could she? Are the whites of her eyes nice and white, or a bit yellowy?
If you think it could be a photosensitive reaction, could you put her and a pal somewhere shady during the day?