This is totally not a silly question. Body language is just as important as the words we say, perhaps even more so.

Do tell your therapist what you observed and how you felt, and ask what was behind it. But don't assume you can work that out accurately without asking - I very much doubt she was bored.

I think it's really great that you posted this question and are thinking about it and what it might mean rather than just stewing on it.

Btw, she might just have had cramp. Or maybe she thought of something she wanted to say but canned it as you were talking and her foot gave away her eagerness to speak.