Ear infections in chickens aren't usually contagious as far as I'm aware but it's wise to check the rest of the flock just in case.
Sound like it's a middle or outer ear infection if there's visible pus and given her symptoms so it should respond well to antibiotics. The only problem is which to use. If you have baytril on hand you could start her on that and see if it helps any? It is a very potent, very broad spectrum antibiotic. If not, see if your vet will sensitivity test some pus/discharge.
if you can stomach watching a vet remove pus and irrigate a chicken ear, give this a watch:
It's pretty graphic if you're at all of a sensitive disposition. Don't say I didn't warn anyone!