My 14 yr old daughter has a 4yr old, female, shepard mix named chessie. We adopted chessie from an animal rescue when she was 6months old. A year later we adopted another dog , Angel, who was 6 months old and is now 3yrs old, female, cattle dog mix. They have never had any issues with each other, Chessie has always been the dominant one, and Angel is more than happy to let her. Three days ago chessie started attacking Angel, even if she is sleeping, when my daughter comes home from school. Chessie greets my daughter at the door then turns around, finds Angel and attacks her. I work at home and am here all day with them, I don't understand this sudden aggression. Angel didn't fight back the first two times and it was easy to break up, but today she defended herself and it was hard to pull them apart. Chessie has been sniffing at Angel during the day and constantly watches her. Angel is now hiding under my bed and is scared to come out even though Chessie is in my daughters room with her now. I am worried!! Any clues to what is going on with my puppies??