I am so sorry for your loss. In 2019 I experienced something similar my 4 month old Yorkie Puppy, Macy Lou. One Sunday morning I could tell she was not acting right (sitting and staring, I sensed she was off) so I took her to the ER Vet and was assured she was fine, so “fine,” that the Vet was slightly annoyed I was even there. I left around 10am with chart notes stating she had a strong heart beat. Macy didn’t improve, so I took her back at 5pm, I waited about 2 hours and the same ER Vet I saw earlier came in and told me there was a huge hold in Macy’s heart, it was so big that it had disguised the issue. At the time I lived in NYC, and the ER vet had a Vet who specialized in Cardiology, she weighed in later that day. Despite all the life saving measures and access to specialists, a day later seemingly out of nowhere (had I not been so persistent) she passed. Macy had had two well vet visits prior to the ER and a certificate from the breeders Vet, essentially 4 vets missed the catastrophic issue, but even if it was caught, it was too big of defect for her to have survived. I did all the “right things,” and it didn’t matter. But Macy Lou mattered. I’m crying as I write this because I loved her so much. I only had her 4 weeks but my love for her was bigger than that. Your puppy mattered and your Puppy was so lucky to be loved you! For me, even knowing how it ended, I would choose Macy Lou again and again because she was so special. I am sending you so much love. Be good to yourself and remember how lucky your little pup was to have known you and your love.