It may be possible she is allergic to those brands. Would you be able to switch her to either Natural Balance or raw meat? Raw food is the best you can give your cat. I would not use it for my cat even though it'd be best to, but he's got bloody issues.
The brands you just mentioned can cause allergic reactions in some cats, thus making them vomit all of it. If you are completely sure she is healthy and has no problems like worms or anything like that, I would change her diet little by little see what happens. Natural Balance is expensive but totally worth buying. Just try it for once and see how she reacts to it. These processed foods with by-products and fillers are really a pain in the neck for some kitties.
Ive tried wiskas, felix, hi life, purina, iams, go cat, she just ends up throwing it up or getting a bad tummy after 1-2 weeks
What brand do you get at the pet shop?
I cant give her kitten food any more she gets an up set tummy
Kitten food, to my understanding has more nutrition. Kittens need calories and nutrition to stay active and healthy. Some brands don't even add by-products to their kitten foods specifically. She is getting good food, don't worry about it. You had her spayed so she'll eventually gain weight unless she gets severely ill (God forbidds). I really think you're on the right track here. You're doing what you're supposed to be doing. Just keep a watch on her and if she either loses more weight or just don't gain any, take her back to the vet. In other words, make sure they addressed the real issue, fixed it and it is all gone now.