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Hello. I'm sorry to hear about your dog.
It's hard to know what happened without a necropsy (autopsy) of the body. With no other signs- ie no vomiting, diarrhea, bleeding- then it would more likely be due to some type of heart disease. This could be a clot (stroke) to the brain, lungs or heart vessels. It could also be a ruptured chordae tendinae (tendon to heart valve) in the heart. Those would be the two most likely causes. There are typically no warning signs, but these dogs can die suddenly. There was nothing you could do to foresee or prevent this from happening.
I'm sorry again for your loss. Let me know if you have any other questions.
Even with a normal EKG, they can still have heart disease present and end up with a ruptured chordae or stroke.
Stress can increase the heart rate, but it won't cause a stroke or a ruptured chordae.