There are times when diabetes occurs secondary to another problem, particularly immune mediated disorders and other hormonal illnesses. Genetic factors, obesity, some medications such as progesterones and chronic pancreatitis have also been associated with diabetes. Diabetes can be a really insidious disease. The weight loss would be a good hint her.
Diabetic dogs do reasonably well until they lose most of their ability to utilize sugar.
Once this happens, they burn body fat for energy almost exclusively. Fat accumulates in the liver leading to liver problems. By products of fat metabolism include ketone acids which cause acidification of the blood and severe metabolic problems. This part of the process does happen very rapidly - often within a few days. If the initial signs of diabetes are masked or missed the onset of severe diabetes can seem to be extremely sudden.