does she do it as the milk is letting down, or a while after let-down? if it's during let-down, it might be that you have a forceful let-down, and in order to deal with the flow, she's clamping down on your nipple to stop/slow the flow so she can keep up with it. (my dd did this in the beginning)
if she does it a while after let-down has happened, she could be getting impatient and frustrated, wanting another let-down (my dd does this from time to time if another let-down doesn't happen right away, especially right now as she seems to be going through another growth spurt).
anytime dd clamped down on my nipple, i would say a very loud "ow!", which would kind of startle her, and get her to let go. (i'm hoping this will work when she gets teeth, too!)
successful #2 Jan. 25th - welcome Maisie Elise!