I'm sorry for your loss . You did everything you could. Liver Cancer can cause them to bloat a day or two before the frog passes due to all of the toxins building up within the body from the Liver shutting down and the frog no longer being able to process them. Kidney failure also happens in a similar way.

There is no cure for the cancer and kidney disease is also almost always fatal to them. I'm sorry there wasn't more that could have been done.

See if your Vet would like to perform a necropsi on the frog to pinpoint cause of death. Maybe he will even do it for free because it will help him learn more about amphibian biology and thus furthering his knowledge. If he does the procedure please return here and let us know what cause of death was.

Once again I am sorry for your loss.