You could wait till she's doing it and say " it bothers me when you do that, am I boring you or something?" Or you could say " I have noticed lately that you seem fidgety ie cleaning your nails etc and I wondered why you do that?

I dunno, it's not so much how you say it but that you bring it up that is important. Tell her how it makes you feel. Your T is a safe person to practice standing up for yourself with. She'll probably secretly cheer you on for doing it!