Why are my dogs fighting?

Okay so I have two great pyrenese... big white loving cute dogs, they have always gotten along and have been together for 2 years. We have a girl 5 years old and a boy 2 years old...lately though they will snuggle outside on this little bed they have bc its cold and every time I go outside one of them wakes up and attacks the other and it goes both ways the boy will attack the girl and the girl will attack the boy.... I don't understand why they are fighting, any ideas?