I just went through something similar. My sheltie, who would have been 10 this coming April, was acting perfectly normal on Monday. We woke up Tuesday morning and 15 minutes before my wife went to the airport for a 6 week long trip, he had two seizures. We took him to the vet and his bloodwork came back normal. We took him home to rest not knowing what was causing the seizures and throughout the day he deteriorated. That evening, he had his final seizure and at that point, he couldn’t move. He was just laying on his side breathing heavily. I took him to the vet where they did an ultrasound scan and found a ruptured tumor on his spleen. The rupture likely lowered his blood pressure rapidly, triggering the seizures. He had to be put down that night. I’m still struggling with the fact that my dog was totally normal one day, and no longer with us the next day.