If you live in a multi-family building, could she have eaten a poisoned mouse (if someone put mouse bait in their unit without your knowledge)? Poisoned bug(s)?
Does she have any injuries? You might have to look very closely to find... maybe got ahold of a rat, bird, snake, squirrel, dog, etc., or got stuck in a very small space, or a tub/shower with a closed door.
Does anyone have access to your place? Could they have given what they thought was a nice treat, but it had carbs?
Dental treat? Cat toothpaste/gel?