We have a white german shepherd bitch who is nearly 10 months old. She is utterly delightful in (almost) every respect; loving, obedient (including off the lead), playful, trustworthy with the children, loyal, faithful, etc.

Recently she has started becoming more and more possessive over her food and treats (namely treats which take her a bit longer to get through) and growls when we get near. I've tested her and she's even barked at me and I've backed away not wondering what she'd do. As soon as the treat is finished she is back to her normal, patient self and we can handle her as before. Any advice? Is it possible hormonal as she's getting ready for her first heat?