My Prince - only about 7 years old - died suddenly last year. The night before he was fine, running around and jumping on my shoulders. The next morning he was lying on his side with blood coming from his mouth. I rushed him to the vet and she couldn't be sure, but she suggested hemorrhagic gastroenteritis - basically they just bleed out internally. He was so far gone we had to euthanize him and she had trouble even finding a vein.
They asked me if I had any poisonous plants in the house. I did have 2 large philodendrons that had wintered inside for years. I'd only rarely seen the cats chewing on them at all, but the vet (a new hire I'd never seen before) snidely suggested that the plants must have been the cause since "we can't think of anything else." Never mind that I couldn't find any mention in research literature of plants high in oxalic acid causing internal bleeding; philodendron causes mouth irritation. That evening, I saw Polo nibble on a leaf. He vomited it up, walked away and that was it. I know the plants didn't kill him. So sorry about your baby. Like you, like all of us, I wish I knew what really happened. Then I might have some peace. When my pets all pass, I don't think I'll have another one. I just can't take it when they die.
"Some of you say, "Joy is greater than sorrow" and others say, "Nay, sorrow is the greater." But I say unto you, they are inseparable. Together they come, and when one sits alone with you at your board, remember that the other is asleep upon your bed." Khalil Gibran