My heart is breaking

My wonderfully sweet adopted dog just snapped and scared my entire family. I don't understand. We were standing there with a neighbor in the kitchen who was about to depart. My husband gave her (the new dog) a hotdog and our neighborbor turned to leave-she was already finished with the hotdog. She suddenly sstarted barking and growling and snapping at the neighbor who had only started to go for the door. My husband went to comfort her, extending his hand, to pet her and she snapped at him! Both he and the neighbor froze. I went straight up to her as her teeth are bared and said, "Diamond what are you doing?" she immediately quieted down. I took her straight out to the garage and shut the door.

Everyone was shaken. A 117 lb. dog baring teeth and barking like a guard dog in a movie about to eat an intruder was enough to destroy all the trust we've built up. I am bawling. I cannot have this misunderstanding, or whatever it was, happen to one of my children. She'd growled at my son this morning when I asked him to put her out. It scared him. Now I am thinking, "what on earth do I do?" I cannot have my entire family being threatened. I know she must be going through hell having been given away by her family, but now I am not sure there is no way my son nor my husband will trust her. Thank god my 5 year old daughter missed it. I certainly cannot ask them to live in fear of her. Nor can I risk her turning my sweet doggie into something like her.

I am going to sleep on it. Anyone have any advice? She had been so perfect before today.