If I understand right she is only fixed and not neutered. This means her ovaries are still working which might be the reason for the aggressive behaviour.
Another reason could be a tumor. Our neighbors had a Dogo Argentino bitch which suddenly turned very aggressive - long story short: she suffered from ovarian tumors and after the surgery everything went back to normal.
Another reason could be a tumor. Our neighbors had a Dogo Argentino bitch which suddenly turned very aggressive - long story short: she suffered from ovarian tumors and after the surgery everything went back to normal.