Causes of Sudden Death in Dogs?

Recently my 8 year old Cockapoo passed away unexpectedly. She passed at around 4 am on a Thursday. She was fine Wednesday until about 6:30 PM when for the first time ever she pooped on the couch while lying down. My husband then took her out and she pooped again and started to sway. She came back inside and went to hide under the table. She looked dead, but my husband got her out from under there. I took her back out and she seemed like she didn't have energy, but she was OK.

When my parents arrived she started to wag her tail, but my dad could tell something was wrong. We decided to have dinner and she sat on the floor by me like usual. After dinner when we were on the couch she even jumped up to sit next to me. I started to give her water and she drank a lot of water. It was more than she ever drank. I called a bunch of vets to see if I should bring her in or if therewas anything to be worried about. They all said that we should watch out for vomiting and changes in her breathing. That evening I stayed up with her and at about 2:30 am her breathing got worse and she pooped lying down again on my daughter's bed. The whole night she kept trying to hide.

I took her to the vet which was about 30 minutes away. My daughter held her in the car the entire time and said that her body just felt different and she was breathing rapidly. When I parked the car she tried to jump in my lap like she always does, but she had no energy. When we arrived at the clinic she was still sitting with my daughter and pooped in her lap while I was talking to the receptionist. As soon as my daughter put her on the bed she had to go since only one person could be in the room. The doctor come in right away and I turned to give my daughter my wallet so that I could comfort Pecan since she doesn't like the vet.

As soon as I turned around, Pecan wasn't breathing anymore. When the vet said this my daughter screamed and Pecan turned her head slightly to look at us. They took her in the OR right away and did CPR and gave her drugs and oxygen. It was no use she was dead. They told us it was probably a heart attack or a stroke, but it's really hard to tell. I truly believe that if we hadn't pulled her out from under the table earlier that night she would have died. I think she was waiting for us to not be there because she didn't want to die in front of us and that's why she kept trying to hide. Does anyone have experience with this? What do you think happened?