My aging dog is becoming a little unstable?

My heins 57 mutt (Pitbull, Labrador, German Sheppard, Doberman, I'm not sure what else) is entering her later years, around age 8 or 9. She's always been a little skitish, sometimes (though rarely) snapping when annoyed and once bit a delivery boy who entered our house, but otherwise sweet and playful. She's also very in shape for her age. We think she's healthy and she has regular visits to the vet. However, she's been a little off the past few months. She is especially wary towards my friends, even when they have been over many times recently. She often looks guilty and yesterday when I was playing with her like I usually do she became suddenly violent, though she quickly seemed to realize her mistake and resigned to her bed. I'm a bit worried about her and how she'll be acting as she gets older. Does anyone have any idea why she's been acting this way? Any advice?