If your cat is 4-5 weeks pregnant then this is probably just morning sickness which some cats suffer for a week or two during this stage of pregnancy. A change in her normal food can also cause vomiting. If your cat is near to full term (approximately 63 days) then you need to take her to the vet. Vomiting is sometimes the first sign of labor but this stage should not last longer than 24 hours. Continual vomiting could be a sign of dystocia (birthing problems) where cat is trying to give birth but the kitten is stuck in the birth canal. It could also be a sign of an infection which only the vet can treat.