yes, cats are protective, they are specially protective when they have just had babies, and they need to protect them. Also brother cats and sister cats protect each other.
They are overprotective because they don't want their babies to get hurt just the same as humans. so yup. There is no such thing as being overprotective when you are a mother cat. but a mother cat...
They are extremely supple and have 34 vertebrates instead of 12 in a human. They can contort themselves because their bones are connected by muscle rather than cartilage.