Why do my dogs fight?

I NEED HELP! I have had my dogs for 5 and 3 years and they have always been best good friends. One is a Shepherd/Boxer mix (Phelos) and the other is an American Staffordshire (Lizzy). Last week they had a horrible fight and Lizzy got pretty messed up. Everything was fine after that until last night. Phelos was sitting beside me and Lizzy tried to jump up there and Phelos pounced on her. The fight lasted for over ten minutes because I could not pull them apart. My son and I finally got them apart, but there was alot of blood and scratches. I was terrified and thought they were going to kill each other. After the fight they were fine and cleaning each others wounds. I don't know why after all these years together they are now starting to fight, but I do not want it to happen again. Does anyone have any suggestions for me????? I am not getting rid of either of them. I cannot do that they are family.