I don't want to worry you too badly but did the vet check her for Distemper?
If it is Distemper, she does still have a fighting chance. Not all dogs make it but some do. Not much can be done for that except nursing care and relief of symptoms.
You really do need to speak to that vet who treated her. You need to tell him her symptoms are now on a decline again, and you need to ask him what he thinks is wrong with her. I would say you should do that as soon as possible.
I hope your little dog will be okay. Bless her.