My name is ***** ***** I’ve been involved professionally with dogs in the health and behavioral fields for over 18 years. It will be my pleasure to work with you today.
The growth you describe sounds like an abscess which occurs when a dog has a puncture wound such as from a bite that bacteria gets into. The outer wound heals but the bacteria continues to multiply under the skin leading to a pocket of infection under the skin. Since you see the scars from bite close the area, I'm pretty confident that is the cause. The fastest way of resolving this would be a quick trip into the vet. He'll likely give antibiotics to help fight the infection and may drain the abscess to remove the pocket of infection. It may come back in the time it takes for the antibiotics to work, but by then the antibiotics are working and the situation resolves itself.
I'd have a good work up done on her as in many cases fighting between dogs who have previously coexisted is due to a medical issue such as an illness that causes the other dog to see them as weaker and thus they might become the top dog. Another medical condition would be hypothyroidism which can cause sudden aggression which leads to fighting as well. Read about these medical causes on the following pages.
Additional training can also help stop the fighting if there is no medical issue. Both dogs should be leashed and if one dog even looks at the other dog, a correction should be done. Any sign of aggression including a prolonged look, hair raised on the shoulders, a growl or even a stiff legged walk, should be corrected. A correction is a quick tug of the leash and a firm low toned "NO". Once you have done this couple of times, you should notice the dogs ignoring each other. When that happens, you will want to reward them for the desired behavior. Again, use tasty treats like the hot dog slices. This teaches the dogs that you WILL not tolerate fighting in YOUR pack.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.