My Female yorkie Lola has started this behavior. She is coming up 4 years old now but has really latched onto my eldest Daughter. The other morning she was happily sleeping on my daughters lap, when her best friend (our male cat) came over to my daughter for a fuss, Lola really went for the cat, chasing him away, she's never been like that with him before. I immediately sent her to her bed for 10 mins to try and cut that behavior out. I hope I did the right thing.
Laura x