what is the underlying cause?

[txffamom]

We made the decision not to do xrays at that time due to her pain level and the position in which an xray would have to be taken, adding to that pain and stress.

Thank you bc, that answers my question. I guess all that's left is waiting on the calf to get here...

 

[branguscowgirl]

We made the decision not to do xrays at that time due to her pain level and the position in which an xray would have to be taken, adding to that pain and stress.

Thank you bc, that answers my question. I guess all that's left is waiting on the calf to get here...

You may never even be able to see a "hairline fracture" in an xray anyway. (Sometimes hard enough to see on people, and they are much less dense than a cow.)
I think all you can do is keep her comfortable, and by herself till she calves. With a pinched nerve, they would give Dexamethesone.
But you can not give it to her now, it would put her into labor. Wait and see approach.... as long as she can get up and down.

 

[chippie]

I think that her back hurts. She may have a pinched nerve or something. Her steps do not appear to be uniform when an animal is lame on one leg.