Why is my dog so aggressive?

My American Eskimo Dog is usually calm and mellow. But today, while on a walk, she bit and killed a wild bird, we pulled her away quickly, so I don't think she consumed any. She was fine all day, but 10 minutes ago, she crawed under our bed ( something she does frequently), and when she came back out, she started acting aggressive when a guest tried to pet her, even though she's done it before, about 3 minutes before this time. Lola (dog) goes back under the bed and refuses to come out. We reached our hand under the bed and she attacked 3 times. We think she doesn't want to stay outdoors because we usually keep her there and she doesn't like it. What is wrong with her?