Aggresive behavior what could be the cause what can i do about it?

I have a jack russell terrier she is about 2 years old. back in january a flood in my house cause me and my dog to move in with my boyfreind and his family and thier two dogs they have a maltise and a maltise/poodle mix. Before i got the dog she had grown up with the maltise/poodle. She had never met the maltise. the maltise is about 13 years old. All of the dogs got along fine until about two weeks ago when my dog started trying to attack the maltise. last week she got into it wih her and bit her in a few places. My house isn't ready i need her to be able to stay here but the family wants to lock her up. I don't understand why they all got along fine since january and now she wants to attack this other dog she gets vicious about it. I love my dog so much and i just want them to all get along. Could this be a medical condition or would obedience school help. I just don't know what to do. Please help.