This is possible because cats have two uterine horns and breed for about 5 to 7 days. Kittens that are concieived at different times can be born a day or two apart. As long as the cat is eating, drinking and nursing the babies she is fine. If she is not doing these things then she should be seen by a vet.
Please let me know if you still have questions. I wil be happy to help.