I'm having the same exact problem with one of my cats (vet appt on Wednesday), but the last time she was throwing everything up, it was fleas. I know that sounds strange for this time of year, but the vet's explanation was that the cat's skin was aggravating her enough (even though we never *saw* any fleas, I have to admit) to make her clean herself aggressively. Hence the vomitage.

If she's been prone to cookie-tossing, she might be more sensitive to whatever's bothering her. Her sibling has no similar problems. Hope you find some answers... cats, unlike dogs, can unfortunately find all the worst places to puke.