Try not to panic. She could have asymptomatic MVD which many dogs live with with no problems.
Here are a couple of good links for you:
Hepatic Vascular Disorders - WSAVA 2006 Congress Portosystemic Shunts FAQ
To determine if you are dealing with a possible shunt or MVD, the next step is a Protein C test. Here is all the information you need to show your vet if he is unfamiliar with it. Many vets are and will just waste your money on an ultra sound which can be as low as 60% unreliable:
Comparative Coagulation -- NYS Animal Health Diagnostic Center