It is possible that the Rimadyl tipped the scales and there was already a preexisting liver/gall bladder issue. The push for the ultrasound is not an unreasonable one - it will let you and the vets know if they are treating the problem correctly or if they need to take a different path.

Rimadyl can cause problems - did the vet run a blood panel before prescribing the Rimadyl to check values (liver, kidney, etc)? Please don't let whatever the problem is go untreated - liver failure is a terrible way to die. The dog will often stop eating, develop diarrhea, vomiting, abdomen can become very uncomfortable, etc. Did they send home a prescription diet? Any medications?

Keep us posted and I hope things work out for you guys and dog.