It could be from dental disease, which would be unusual for a young dog, unless she has retained deciduous canine teeth (baby teeth that never fell out), and there is a bit of blood around the base of these teeth. A traumatic injury to the mouth that has caused some bleeding could cause this also.
Bad breath can also come from further down in the digestive system: the stomach, intestines. etc. If there is no problem with her mouth try giving your dog some probiotics in the form of Activia yogurt, and over the counter digestive enzymes, such as papaya enzyme.