I've had several cases of Listeria over 30 years. It is a soil born organism - hence its presence in haylage that has soil contamination, but can be contracted from the soil alone as in the muddy conditions so prevalent at the moment. In sheep you need a larger than usual dose of antibiotic to cure it and it needs to be done as soon as possible. I would therefore give another (large) dose of pen strep today, although tetracycline is better if you have it. Even if it is a long acting antibiotic, you need to give it daily to keep the level up.
By the way - I understand Harmony's reticence in giving antibiotics for everything whether diagnosed or not, because of the real danger now of antibiotic resistance. However this animal is presumably not going into the food chain, or in fact going anywhere else, and if she in fact has listeria then to have delayed giving her antibiotics last night would have probably meant it was too late as it develops very fast in sheep if untreated.
As for the possibility of a mineral deficiency - I would get a good general purpose sheep vitamin/mineral drench for her anyway. It won't do her any harm and is always a useful thing to have in for when any of the others look off colour.