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Thank you for the reply.
In most cases of sudden death, there are only a few disease conditions that would progress that quickly. One is a cardiac abnormality that would lead to a fatal arrhythmia and cardiac arrest. Boxers are a breed that are over represented for cardiac disease, and it can go unnoted until something like that happens. Another possibility can be a pulmonary thromboembolism (blood clot to the lungs). This can occur secondary to cardiac disease as well as other metabolic diseases that may have gone undiagnosed as well. Finally, a probably more likely due to her age, is a ruptured abdominal mass. This is usually a tumor of the spleen or liver that has been slowly growing (with no symptoms) until it gets large enough that it ruptures open and bleeding into the abdomen occurs. These are very vascular organs and these dogs die fairly quickly.
With the symptoms that you describe, I would think that any of these may have been possible. If so, it is very feasible that she would not have shown any signs to alert you any earlier and even if you were able to get her to a vet quicker, it is unlikely it would have had a different outcome. It is hard to lose a pet without warning, but hopefully you have have some comfort in knowing that she didn't suffer.
Please accept my condolences.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.