Only info so far is they found nothing wrong except for bleeding in his intestines. No cause. Other than bleeding to death in his intestines he was in perfect health. They are doing more tests but it could be auto-immune.

Stewie just started passing bloody poop in the evening and he bleed out over night, remember E-vets wont see them Rushed him in before the vet was even open and got him in next morning but they couldn't save him.