I don't know anything about pregnant cats, others here have experience and can answer if that is part of the problem.
I do know that any sudden changes in diet can make for upset stomachs and vomiting in many cats. Not all react that way, but many do. If you have recently changed her food altogether, go back to the food she is used to and switch her over gradually. Mix the food about 3/4 old to 1/4 new at first, then gradually add more of the new food to less of the old. It is best to have the switch over about 2 weeks or more so her stomach can get used to the new food.
Hope this helps!