It's a vitamin deficiency.
I've had this happen before too. It's like she can't hold her head up right?? You need to get some polyvisol without iron from your local pharmacy-you'll probably have to ask the pharmacist (they like to keep it behind the counter). But make sure it's the kind without iron. It's about $10. What I do is fill the dropper with the Polyvisol then take an equal amount of water and mix them together. Then just put it in her mouth and get it all down her...take your time with her. It usually takes about 36 hours for the vitamins to completely kick in, but she'll get better. If you can't get the polyvisol, you can try the powdered Durvet vitamins, but I never have, so I don't know if it will work. But the polyvisol works for sure!
Good luck!