What causes a VERY happy VERY loving VERY healthy cat to all of a sudden start attacking?

We will just be loving her nicely and she will just lunge and bite and then when we do get our hands back she growl and hiss and spits at us! She has all of her needs met such as fresh food and water, a clean potty box and LOTS of love and attention. It is a very calm environment and not a lot of visitors. Does anyone have any suggestions? Thanks for reading.