So... lots of research. This one seems much more likely to have "grass staggers". The paddock was really rich, but its a new area I haven't limed. Lots of fresh sorghum and stuff growing fast. The cerebellar symptoms- head bobbing, eyes ticking back- and forth- fits the bill. This is insane, a completely different cause, but literally in the same paddock (out of 24 in the rotation).
Per the internet (so you know its true), this is common with fast growing pasture, since they don't store much magnesium. They had free choice mineral!
Well, either she will get better or she won't. The cure apparently is to inject magnesium and calcium subcutaneously. They don't give a dose, and I don't have that lying around. She is able to move but she isn't active. I am going to give her some multivitamin/mineral gel tomorrow and some Dyne or something.
Just odd- same sheep was grazed on newly established pasture for the last two years with no issues. I was concerned this was due to the use of sorghum (not sudan, just regular sorghum), but its what we use to establish pasture all over here.