What is a very fast, natural death for a young character?

I have an apparently healthy character in her 20s that I need to kill. It has to be very fast and without warning (even to her), and I'd prefer if it were natural, instead of something like getting shot or hit by a car. The whole point is that no one saw it coming and there's nowhere to place the blame since the cause was internal.

In the scene I have in mind she's walking to her car after a late night at work, and then she's dead. Any suggestions?

She could have had an aneurysm - which is a weakened blood vessels or unusual bubble in a blood vessel - burst. They can be present from birth, but not detected, because aneurysms in certain areas of the body (like the brain) show no symptoms, but once they burst, the risk of dying is very high.
Here's what PubMed has to say about brain aneurysms: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002387/
Hopefully that helps?

It could be from Sudden cardiac death -- it's a natural death from cardiac causes, heralded by abrupt loss of consciousness within one hour of the onset of acute symptoms.[1] Other forms of sudden death may be noncardiac in origin. Examples of this include respiratory arrest (such as due to airway obstruction, which may be seen in cases of choking or asphyxiation), toxicity or poisoning, anaphylaxis, or trauma.[2]heralded by abrupt loss of consciousness within one hour of the onset of acute symptoms.[1] Other forms of sudden death may be noncardiac in origin. Examples of this include respiratory arrest (such as due to airway obstruction, which may be seen in cases of choking or asphyxiation), toxicity or poisoning, anaphylaxis, or trauma.[2] wiki

I believe Reggie Lewis, an NBA player w/ the Celtics, died of this.

You'd have to do some more research into this, but an embolism or blood clot could be shaken loose by something relatively minor. Fat embolisms can occur after a bone break (if it's possible, being able to say, 'She broke a bone four months ago, and that's why it happened' could lend a sense of legitimacy). I don't know how long that could sit around inside the body. http://en.wikipedia.org/wiki/Embolism