Its a little bit cheating seeing as how toxicology results have been released, but I do this game at work all day every day soooo, here is my guess as to what killed her.
Hypokalemia. I figure since she was a skinny girl maybe she had a problem eating, or abused laxatives for weight control. Maybe excessive drinking (heavy diuretic effect, or just poor general nutrition). I assume since no illegal drugs were in her system, it had to be an electrolyte imbalance of some sort.
Cardiac Arrest due to electrolyte imbalance/metabolic disturbance secondary to alcohol withdrawal and malnutrition/drug abuse.