I had a toy schnauzer that was 9 lbs 1 yr.old die from taking tribrisson.She did not have systemic shunt. She was very healthy never sick until she was around a dog a neighbor had that had a runny nose. The Vet prescribed the tribrisson. The Vet did all kinds of tests but couldn't save her. The meds affected her liver. I know this is suppose to be rare but found out some other Schnauzers have been affected by this drug too.
I am so sorry about your sad loss!
Your dog could have appeared normal in an exam when she went to the vet on Jan 19, but there may have been some abnormality that would only show up with more involved diagnostic procedures, such as blood work, ultrasound, etc. Some abnormalities could be microscopic, such as some forms of cancer, and would not even have shown up on X-Rays, Blood work, ultrasound, MRI, or other diagnostics.
Other cause of sudden death may include: toxin ingestion, internal wounds from trauma, electric burns, bleeding disorders, lightening strike, gastrointestinal torsion, lung torsion, heart worm disease, heart failure, splenic tumor that bursts, Lyme nephropathy and many many more can also cause sudden death. Only toxicology screens, blood work, a post-mortem exam and other diagnostics may be able to determine the cause of death.