I think you figured out the most likely cause- she challenged the older hen.

The older hen is exhibiting perfectly normal behavior in this situation. It becomes a "problem" only when the loser cannot get away from the victor or somehow gets cornered.

It is possible to remerge her, the old hen will want to be extra bossy on her for a while though so better let them interact freerange (if you lets yours out of the coop?) so she can chase her around to her heart's content and she will ease up on bossing her around so hard eventually.

One thing though.. if it all came about because she did challenge the older hen, if you keep her isolated until she heals, she may regain the courage to challenge the older hen upon re-introduction. In this case, if you let the hens freerange I would recommend letting all out at once soon- as in tomorrow or 2 days later(and close the coop so she cant get cornered in there should she go in while you aren't out to observe). Her defeat would still be fresh in her memory and she should not try to challenge her again this soon.