I have a magnificent two year old giant schnauzer, who has always been very friendly around people and other dogs. She's normally a very family orientated dog, and has always been treated as one of the family. Last night, we were sitting on the couch, and "Balloo" came up and layed up beside mum, which is very unusual. My little sister then came back to take her spot, where Balloo was very contently laying. She sat down behind the dog, and layed lightly across her back, to be greeted with a menacing growl, after getting up and repeating it four times Balloo freaked out, she turned around and basically attacked my sister. After a good scolding from the fam, and being locked in the mud room for the night, she seems fine today, but were frantically worrying about this incident.
She had no change in diet, though she did get a dog bone, and devoured it, (she never gets them) so Im wondering if it was hormonal properties maybe? I examined her today, (having veterinary experience myself) wondering maybe if she had hurt her hips prior to the incident, as she is 115 pounds and very energetic. But no such injury was detected. However, I did come to find that she is in her heat cycle. (Spaying appointment has been made) so dominance is also a concern.
She is an extremely gentel pup, - she even plays with our baby rabbits, she lets them climb all over her, she cleans them, and never once has she injured, or killed anything. We also spoke to a relative who has a wirehaired dachshund, Finnegan, who also gets agressive when he eats this particular type of bone.

Any Ideas???