Help!

My baby girl is 3 weeks old today and I have been breastfeeding, pumping and using formula. When I'm not nursing and we use a bottle we always put 3 oz of either breastmilk or formula in the bottle. She will eat the bottle and then still be hungry. At one point I left to run errands and my mom gave her two 3oz bottles of breast milk within an hour! Or I will give her 3oz and then she will nurse! Is this just a growth spurt?? Should I be doing more than 3oz? Obviously I will talk to her pediatrician at the next appt but just want some ideas until then!