Does your vet have any injectable antihistamine to try?

If not, could try a dose of dexamethasone (since is not pregnant and already on antibiotics)

Third choice would be regular dose of flunixin meglumine.

None of these may help but they also won't hurt.  Symptomatic treatment for allergic type reaction.  Good luck. 

(Not blackleg inasmuch she would be dead by now.)