My baby sucks on fists, I give breast - refuses, then suckles a pacifi…

My baby sucks on fists, I give breast - refuses, then suckles a pacifier and throws it away and starts sucking fists again. How to stop her? Why does she do this? Share with anyone who had this ? P.S. We're undergoing treatment, I give her a bottle of water and Duflakom often, glucose three times a day with a syringe.