Well this is strange. Your vet did an ultrasound, told you that, and then didn't bother to give you a diagnosis, or any suggestions for treatment??
Doing an ultrasound then leaving you wondering like this isn't veterinary science, it's photography! Ask the vet about this. If you aren't getting satisfactory answers, go to another vet.
Your dog's gallbladder is probably inflamed, or obstructed in some way. Gallstones could be the cause. Other things can cause this. Infection can. E-coli (normally present in the gut and a natural 'good bacteria' in its rightful place) can sometimes 'back up' into the gallbladder, and cause infection there. Sometimes muscle contractions (to expel bile, necessary for digestion) can fail. There are quite a few different causes.
The Content on this Site is presented in a summary fashion, and is intended to be used for educational and entertainment purposes only. It is not intended to be and should not be interpreted as medical advice or a diagnosis of any health or fitness problem, condition or disease; or a recommendation for a specific test, doctor, care provider, procedure, treatment plan, product, or course of action. MedHelp is not a medical or healthcare provider and your use of this Site does not create a doctor / patient relationship. We disclaim all responsibility for the professional qualifications and licensing of, and services provided by, any physician or other health providers posting on or otherwise referred to on this Site and/or any Third Party Site. Never disregard the medical advice of your physician or health professional, or delay in seeking such advice, because of something you read on this Site. We offer this Site AS IS and without any warranties. By using this Site you agree to the following Terms and Conditions. If you think you may have a medical emergency, call your physician or 911 immediately.