I am so sorry for your loss. My entire family is dog lovers and I know it's hard when you lose a friend. My rat terroir/chinuanua mix is like my daughter. As for your question it could have been a seizure. It also could have been many other things. My grandmas chinuanua had something wrong with his heart. They only figured it out after he had passed.
I would say stop worrying about the why, and just remember what ya did together. The reason behind her death doesn't matter. My grandparents beat themselves up for a long time, after their 12 yr old chinuanua mix died suddenly. They always wondered if their was something they could have done. It took forever to get them to stop blaming themselves.
Once agian sorry for your loss. Just don't beat yourself up over the why, instead remember the joy she brought your life. I am praying for you.
That is so sad and, obviously, so unexpected.
It sounds like you are not aware of any health issues she may have had?
As in people, could be any number of things like heart problems, an aneurysm, tumor of some sort, etc. unless you have a Vet do a necropsy to try to determine the cause, you will probably never know with certainty.
Take some comfort in the fact that she seemed to be happy, active and well loved. Rather she end her journey in this life that way then to suffer from some awful disease.
My heart goes out to you.
Hugs