Heart Attack?

[steve]

You may wish to have a look at the rear end of your bird, pay attention to the oil gland. If that is swollen and appears to have white substance inside, then your bird has suffered a stroke. If it is not then it seems to be a heart failure.

As far as I could see the bird had no abnormalities, she was in good condition, lively and very active.

Steve, how old was she? A young bird is unlikely to have a stroke.

She was a young bird, had not even bred yet, let's say about a year old.

Regards
Steve

[afinch]

[she] had not even bred yet

That's natural selection for ya.