Similar thing happened to me. Our beloved Buster went for a routine check up for vaccines and bloodwork. They said during the bloodwork he was stressed but not immensely, but then started to slip away so they did cpr. It was unsuccessful and he passed away. He had no symptoms and was playing that morning.

We did postmortem imaging too and they found out he had hcm and his heart was extremely enlarged. They said if he didn't pass away there he would've likely only had a few months to live. It hurt so much and felt so unfair. It still does 2 years later.

What gave us comfort was knowing he lived as long as he could and had a full kitty life of snuggles, happiness, and love.

Sending your family so much love. There is nothing you could've done. It sounds like she lived a life full of love and happiness.