Hello, I'm Dr. Deb. I'll do my best to help you today.
First, let me extend to you my deepest condolences on the loss Lady in this way; how traumatic and heart-breaking for you and your family.
Of course, without an autopsy, we can't know for sure (and sometimes even with one) why she passed in the manner that she did but there could be several reasons for her sudden death
1. heart attack (or fatal arrhythmias or conduction issues) as you suspect. This is probably the most common cause of sudden death
2. pulmonary thromboembolism (clot in the lungs)
aneurysm (rupture of a vessel wall).
4. possible clot or embolism to the brain
5. rupture of a splenic tumor which might have been undetected
6.Ingestion of certain human medications, toxins or poisons although gi signs are usually seen...so this seems less likely to be the case here.
In most cases of sudden death that I've seen, there were no outward signs that the patient was ill in any way. Most of them were otherwise healthy and happy. The majority of these dogs don't appear to suffer either although I'm certain that there's some momentary confusion about what's happening.
I know in these situations that the tendency is to feel or think that we missed some signs that our pet was ill; that if we had only intervened sooner, that there could have been something to prevent the horrible outcome. But, this isn't true for most cases of sudden death so I hope I can provide some reassurance in this regard.
Again, my sympathies to you and your family at this sad time but I hope this helps at least in some small measure. Deb