It is really hard to say. A vet will tell by her weight, size and teeth. If she still has baby teeth then she is under 7 months. And yes she can be pregnant at this age. Even younger in fact. It is not a good idea to let her have the babies this young though, she is not mature enough. You should take her in to be spayed whether she is pregnant or not. Then she will not have the babies and there will be no risk on her life and health and that of the kittens. And you also won't be worrying about bringing unwanted kittens into the world and have to spay and neuter and vaccinate and rehome them all, assuming they are all healthy since you have no idea the condition of the tom cat she mated with.