What Caused My Dog To Die?

On sunday afternoon i noticed my dog was not very active as she usually is.monday morning she did not want to eat or drink. I took her to the vet and they said she had whipworms and sent me home with meds.on tuesday night she died. Her symptoms were lathergy heavy drooling and slime in her mouth ,some vomiting and projectile diarrhea. On the day she died she would shake her head like she was having a seizure. The doctors still dont know what caused her to go so fast.