Some mollies are just like that. Are you sure it's a female? IME fighting usually occurs between males. Balloon mollies do seem to be a bit more aggressive than regular mollies.
One a side note, balloon mollies are selectively bred with a deformity, short bodies and curved spines, which is why they have the unusual shape. They are prone to swimming issues and other health problems, and usually have a shortened lifespan. I feel they are a very unatural creation, and because of this I encourage other hobbyists to keep regular mollies. Typically a breeder would actually kill a deformed fish to prevent it from breeding, and what has been done with these balloon mollies is the oposite, and I personally feel it's unethical. Just wanted to let you know.
