have you tried to talk to her outside of the situation and ask her what she's thinking when she meets someone new? Sometimes talking about it outside if the situation can make all the difference.
See if she can come up with a different way to signal you when she feels uncomfortable. B/c she should be able to feel that you are on her side, ready to protect her from her perceived danger, instead of embarassed by her behavior. That's the only way she has of expressing that feeling.
Then ask her if there's something you can do in those situations to make her feel better. You two could come up with a secret signal, like petting her hair that tells her she's safe.
Is it just in the grocery store (total strangers) but also with family members?
I would also consult a therepest.