feeding question.

Ok other mommies I'm going to call Baby's ped tomorrow but just to hear from you guys. She is three months old. Just turned three months actually and formula alone isn't satisfying her anymore. She will eat 5 ounces at every feeding but no more so we can't increase her formula intake but she's getting hungry like an hour and a half to two hours after she eats. I know everything says to not introduce cereal until 4 months unless other wise directed by the ped but has any one else dealt with this and how did you fix it?