Give her more to eat if she's hungry. My LO was eating 5oz of BM from a bottle by the time he was 6 weeks old and he still eats that much. Try putting 6oz in the bottle and see if that does the trick.
It's really not recommended to put rice cereal in the bottle unless the doc prescribes it for reflux. It's just a filler and if your baby is really hungry then she needs the nutrition, not filler.