shih tzu concerns

Question
I have a 8  month old shih tzu.  I have taken her to the vet twice and the emergency vet center once.  She is unable to stand and falls down if she does rise.  I have been given her water by a dropper, but she stopped eating yesterday.  Once she drinks she starts gagging and it becomes thick and foamy.  She keeps most of the water down, but seems to have a hard time breathing.  THe vet says that her breathing is fine and even though she sounds raspy/rattley, her lungs are clear.  Have you ever heard of anything like this?  I am extremly concerned, and am not sure what to do for her, we love her so much and we don't know what is wrong.  Any help would be greatly appreciated.  Thank you!

Answer
Vicki - Has any one checked for Congestive heart failure -  The breathing would be gagging and she may have a congested sound.
She is young for this, but you never know.  Ask them to take a photo of her heart ( x-ray) to see if it is enlarged)... They can also get an x-ray of the lungs.
What have they done//// any x-rays?
What if any blood work?
Let me know