I normally give the capsules, but if you have to inject it you do it in the breast muscle, in the middle of what would be the thickest part of a chicken breast from the store. Not deeply, just so it goes in. Penicillin is thick so go slowly....
I would try putting her on an antibiotic like penicillin or amoxicillin. Amoxicillin can often be found for pet fish in capsules. I' give one 250 mg pill for standard size hens. In penicillin injectable, I believe it's 1 cc
Take a listen to her lungs, under a wing, listen for rattling, wheezing etc...? Any water or puss in her nostrils? Any bubbles in her eyes or watery discharge? Open mouth breathing can also be a sign of pain, but if you hear noises in her chest then it would be respitory. No stck egg? Is...