Propping her up on her brisket is a good idea, let the rumen have some space to work.
It could be one of a gazillion things, the only other thing I'd give one of mine is a B-Vit jab, as some of the gazillions benefit from that as well as the a/bs. I assume she's got hay within reach, nd some water.
I think there would be a temperature if it was septicaemia, so hopefully it's one of the things that the a/bs will hit.
Bless you for taking care of her. What a good neighbour.