My mom has stage 4 metastatic cancer of lungs, but primary is unknown. Her gallbladder mass, ca is 19.9 which is high but her liver is alright. She got a CT scan done three months ago and her gallbladder seemed fine. Could it be gallbladder cancer?

Gb unlikely. Your mother has stage IV cancer with mets to lung. I am sorry. If a ct showed the gallbladder normal 3 months ago then it is unlikely. Knowing if it is adenocarcinoma or squamous may help. But focusing on the forward progress and implementing a plan she wants is the most important thing. A supportive family and a clear goal. Hope this helps.
Question unclear. A ct scan is not always the best way in which to image the gall bladder, but this is not the most common source of this condition. Right now, rather than worrying about the primary site, your mother needs all the help and support and loving possible. Before making choices of therapy would always ask what to expect and how it will influence life style and what are side-effects. Look into hospice.
Best to use ultrasound. In evaluation of a gallbladder ultrasound is more sensitive than ct. Would recommend reevaluation of the gallbladder with ultrasound.
Possibly. Sorry to hear of this.If 90 days ago there was a normal gb on ct it's unlikely that this is the source. Your oncologist will guide you in determining management.If the gb is the source there's no advantage at this point to remove it. Good luck.