Whats wrong with my dog?

I got my dog at a shelter and had her for about 2 years we were told she was 2 years old.. all of sudden she's been breathing heavy and shaking also falling over to her left side when she shakes or tries to jump..this has been happening for about 3 weeks.. took her to the vet they took blood work and the stuff they tested for came back normal..all they saw was she was a bit dehydrated.. shes a lhasa apso mix weighs about 30 pounds usually very active and loves to jump on furniture and people but know she shakes wen she even "stands up" or tries to jump. ...what else should i get her tested for. or what else could it be?