My 8-year old Pomeranian passed away yesterday due to unknown causes. He started vomiting a clear, mucus-like liquid for about two days before and after searching him we had discovered that he appeared to have some pain in his shoulder. He drank a lot of water and would almost regurgitate it as opposed to actual vomiting. He started having seizures about an hour prior to his passing. His vital signs were taken the day before and they appeared to be normal. Yesterday, his white blood count was slightly elevated and we took him to a specialist that was four hours away, but nothing could be done to save him. He died five minutes after we arrived. The vets tried to revive him to no avail. The vet had indicated that she had never seen anything like this before. This dog was under my constant care - he did not get into any poisons, plants, or otherwise. He was always fed premium dog food (Fromm) and was given filtered water. I am baffled and so is the vet. Please help.

Optional Information:
Age: 8; Male; Breed: Pomeranian

Already Tried:
General blood tests and x-rays, which revealed no abnormalities.