Like Humans.., They are Social Beings!
Well, my cat has been chased in by our other cat. so it is possible for that to happen to yours.
I'm guessing a raccoon.
Always make sure she has plenty of fresh food and water since feeding the kittens and the birthing process in itself can take up a lot of energy. Providing some cat milk and treats can also be good...
they hide in a trash can n................