My cat, Ginger, used to do this several times a week no matter what she ate or how often or how the food was given. The vet, after checking her out several times, said he didn't know what the problem could be. She lived 22-1/2 years. As others have said, some cats just do that. She also used to breathe in a funny way, rapid and shallow, and that always worried me, but it didn't hurt her at all.

Just keep an eye on her for any other changes or problems, anything in her vomit such as blood, excessive slimyness, etc.