My Sister had a beautiful gentle bull-mastif. He was great with everyone and well known around the area. He accepted pats from all kinds and she never had a problem with him until one day when he was about 8 years old. (which is a good age for that breed) On that day he snapped at someone who came to their front door. She thought it was weird because he had never done anything like it before but put it down to maybe that person was a bad person or something. Quite a while later it happened again, I can't really remember the details. Then it started to happen more regularly. My Sister ended up taking him to the vet where they did xrays and found that he had a tumor on his spine. Unfortunately there wasn't a good outcome in this case but maybe you can ask your vet to give her a good checkover. I hope it's nothing serious in your case.