One of my friends runs a great dane rescue, close to me. One of the biggest reasons she has danes returned is exactly what you are describing. They are great in their foster homes, but start to growl, guard and eventually nip, in their forever homes. She is challenging you right now and just testing the waters to find out what her boundaries are. Some dogs will challenge, periodically, throughout their lives. They are IMO just very dominant dogs. I don't have a problem with that, it just means that I can never let anything go. If I say do something, they have to do it, or I have to force them to. No second chances, no "it's okay this time", ever. ChiChi challenges us, even though we have a multi dog household. Part of her problem is that she doesn't really fit into the hierarchy here, since she is the only female. She kind of has her own totem pole, so to speak. You just have to meet her, move for move. If my dogs growl at me over food, they have to eat it out of my hand. If that means I physically struggle with them to take the food away, so be it. I have only done that twice. They like being able to lay down and hold their food in their paws and eat, so to have to stand up and tear off little pieces, while I hold it, is punishment for them.
Just stand up to her and you will win. Don't be apprehensive or worried about her reaction. You will transmit those feelings to her and it will be a losing battle. Good luck and you can win.
Kathy