I'm afraid I know nothing about sheep

and could be way off the mark, but it's just a thought.
Could it possibly be stress related - as in alopecia in humans ? or perhaps some kind of deficiency leading to the hair loss ?
If she seems well in herself and is eating it might not be anything to worry about - maybe even a side effect of the antibiotics

I'm sure some more experienced sheep owners will be able to help more than me, good luck with her Chris