Sorry, I meant a reaction to the medication used for the worms!
And yeah, testing for allergies is hard when you go by trying different foods, their are so many things that could be causing the allergies, not to mention she could be allergic to more than one type of food which just makes it more complicated!
Just some common food allergies in cats that you could try cutting out:
But remember, it takes a couple days for things to get better when there's an upset in the digestive system, so hopefully your vet can help you a little more with that.
Also, when you were calling shelters did they think you were trying to give your cat up? Sometimes that's what shelters assume, and while they shouldn't just ignore you, they just don't have the resources or time to deal with another cat coming to them. If you explain that you don't want to give your cat up, you're just looking for a more affordable spay and vaccinations, and if you explain the tight bind your in they might be more reasonable. If they still ignore you then that's another issue entirely