It should not be an allergic reaction if it is the first time contact.Do you know if some chickens can be allergic to the antibiotics? First day of antibiotics today
But she might experience the effect of the toxins produced by the dying bacteria. Keep her hydrated in case she does not drink enough on her own.
When in doubt, contact the vet that prescribed the antibiotic.
There might also be something else going on with her.
I hope they will all recover soon.