Could be! They do lay a whole clutch before going broody so she might be building one up for herself. The hanging back from the others and being defensive of the nest site could also be indications that she's thinking about it.
oh good! She has about 9 eggs in there now. She is stealing everyones eggs. I have 2 other laying Duck hens so there are 3 eggs added a day. I figured she would start sittign at about 16 eggs.