Bellatrix was testing her. She still wasn't convinced that Hermione would be able to betray her old friends in such a manor. She was ready to push Hermione into the deep end and see her swim. If she could do it, she would be quite a beautiful prize piece. As they left into the main courtyard and ready to somewhat teleport in flames to where they wanted to be, Hermione became stuck  in her own head. She knew what she had to do and why, and with most of the enjoyment she had had here she wasn't sure why she was starting to have doubts. Subtle, but they were there.