What did the vet inject?
Sounds like something neurological involved with head thrown back and glazed eyes, possibly thiamine deficiency (ccn) so if that was the case I would want to give b vitamins, which your vet may not have done if diagnosed with straightforward pneumonia earlier. It could also be a low blood sugar from stress and illness so things like flat Lucozade or syrup solutions would help there. I'd get the vet to have another look asap from the symptoms you describe.