Why does my cat randomly vomit?

We got our cat from a rescue shelter near our house. She was 2 when we got her and she has already had kittens and been spayed. We took her to the vet when we first got her and they told us that she has feline . I guess this means that she can have problems with a runny nose, watering eyes, etc.. but can that cause her vomiting? Sometimes she wont vomit for weeks and then all of the sudden she will vomit over the night. We feed her dry food. Sometimes her vomit is very solid like she didnt chew her food and sometimes its more digested and mushy. Its almost like she is eating too fast and throwing it up. We were going to try wet food but we have another cat and he is on a pretty good diet and likes his food (nutro). We though hers was food alergies so we tried all of the natural foods but the stuff we have had the best results on is friskas (I think) mixed greens. I know that stuff isnt good for her either but she doesnt vomit as much on it. She is also a smaller cat, so maybe we should feed her kitten food so its easier for her to chew? I am very confused on what is wrong. Anyway what do you all think?