My Clawdette passed similarly. Sadly, she did have signs of distress, but they lasted for a brief period. She too was not old. And had been the picture of health prior. Then suddenly was gone.
My vet said sudden heart failure. Rare in cats, but there.
I am so sorry for your loss. I know pretty much exactly how you are feeling, though. I truly do not believe one is ever prepared for such a loss - no matter how long we have seen it coming and known that such would be the inevitable outcome. But the sudden loss truly adds an additional spike through the heart. Because then one can't help but ask over and over - was there something I missed? Was there something that should have been done, but wasn't? Was there......
There wasn't. Not for me and Clawdette nor for you and Teddy. Please reassure yourself with that.
She had the best death anyone can have. At home. Beside the person she loved. At peace. Warm, secure and loved.
And know that you are loved in your loss.
By her. By me and my herd. And by the Purrs family.
Much, much love...........