My preemie is almost 3 weeks old

I had my daughter at 35 weeks 4 days. She spent some time in the NICU. She will be 3 weeks old on Tuesday since I stopped breastfeeding due to mastitis, she literally wants to eat almost every hour. It’s almost like she isn’t getting satisfied when she eats. I am exhausted an have two other children that never done this. Is this normal? She eats anywhere from an oz to 4oz just depends.