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.
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