It's a modern-day story, set in the USA, and my character is in her late twenties.
The only thing it has to be is that it's something that kills her immediately.
Beyond that, ideally it would be based on a pre-existing condition that she knew about, but the likelyhood of it manifesting were tiny, so she wasn't really bothered about it. She drinks a decent amount of wine, eats a lot of rich food,(and would be very unwilling to give these up, except on the very strictest of doctor's orders) and used to smoke, if that might affect things.
I want it to be something that no-one saw coming, but they were loosely aware it was a possibility.
Googling has mainly got me warning pages about taking care of my own health. Which is lovely and all but not best helpful.