I have a neutered male cat age 3. I brought home a 3 month old female and he fell in love with her, never took his eyes off of her. She was spayed at 4 months. Together they played and had a great time - he was happy to have a friend and was always cleaning her and watching out for her. She is now 1.5 years old and my male cat is about 4 years old. He has started fighting with her and biting her on the back, never lets her sleep on the bed, eats all her food and acts very territorial. I don't understand what changed. They were happy for a year - now not so much. She stays clear of him as he is quite big. Would anyone know why?