15m old pinching breast

My 15 m old is still breast feeding, but recently when she’s nursing on one side she puts her hand in my shirt and goes for the opposite nip and just pinches it, plays with it, and sometimes she’s really rough. No matter how many times I tell her no, or move her hand she keeps doing it. Idk why my mind went to maybe my supply is decreasing and she’s trying to squeeze more out of me with her hand?? I don’t know why else she would start to do that. Has anyone else had their baby’s do this?