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Can you give us more info on his eating and drinking habits, defecating, is he lethargic? More info can help us to give you a good answer.
Continuous throwing up can be caused by a number of things:
It could be caused by hairballs, you can try switching her to a hairball control formula food. Several companies offer them on the low end is Science Diet Hairball Control formula food that starts at $7.59 for a 3.5lb bag at http://www.petsmart.com or at your local pet store or you can try Dr. Foster's and Smith brand Advanced Hairball Control that starts at $7.49 for a 3lb bag at http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=10076. There are also many hairball remedies you can add to your cat's existing cat food and can be purchased at you local pet store.
It could also be caused by Feline Irritable Bowel Syndrome. This site offers great info on this: http://www.moggies.co.uk/html/ibd.html
You should just keep an eye on her and if you see a decline in her energy or if she seems lethargic then you need to get her to a vet ASAP. Also, you can check for dehydration by pinching the skin on her back of it returns to normal quickly the sshe is ok but if the skins takes a while to go back to normal then she is dehydrated and should see a vet ASAP for rehydration.
Hope this helps!!