I have a 11 year old Lab/Chow mix in the past year she gradually gained weight, more towards her stomach area and her back paws got a little weak and I thought it was just due to the fact she's getting older. Than for a few days she started to get real sick, breathing weird and her back paws started giving in on her and vomiting and she only would drink water. I took her to the vet where they took x-rays and blood and he didn't find anything. She stood overnight with an IV in her and when I went to pick her up the next day she seemed a lot worse couldn't even walk her back legs where gone and the vet suggested to put her down but I just couldn't do it without knowing for a fact she wouldn't make it. I took her home 2 days ago the first night she ate a little chicken and broth and continued drinking water and at night her breathing got worse and she seemed very uncomfortable. The next day she appeared to be better her back legs started working a little more she went out to urinate in the morning and later that day she urinated and pooped. She wouldn't eat and she started vomiting like saliva. Can anyone relate to this or maybe have an idea what it is??

At first it sounded a lot like the tumor my 13 old Lab had early this year. The vet should have caught a tumor. All I can say is stay in touch with the vet. You may even want to check with a different vet.

My initial thought while reading the post was a tumor too. I agree with labman. If you like your vet, I would suggest going back and insisting on more tests. If you have lost your faith in him/her since this last visit, search out another vet as soon as possible, for a second opinion. I know the last thing you want is for your dog to suffer. If I were you, I would make myself a pain in the rear and wouldn't stop asking questions and searching for answers from a vet. People online can make guesses and tell you all about their own experiences, but without being able to examine your dog, you will only be left with possibilities and won't have be any closer to an answer. I would hate to see you instructing the vet to look for something that may not be the answer to her problem. It will be costly in both time and money. Please keep us posted on how she is doing.

If her knee joints are going bad, even if they are not since she is an older dog, you might try giving her some Hylaronic acid. Wal Mart sells it. I take it to keep my joints from going bad.
I gave it to my last dog when she got old and couldn't climb the stairs. It helped her good enough to help her climb the stairs. It takes two to four weeks to kick in. Vets give it to race horses.
If the problem is a tumor or something that is pressing or whatever there are other things you can give her but keep up with the doctors diagnosis and input, etc... on everything.