My Six week old daughter consumes 5 oz of formaula per feeding...?

She was born at 9 pounds 6 oz. and now she is 6 weeks old and weighs 12 pounds and is 23 inches long. She takes in 5 oz of formaula every 3-4 hours..but lately she hasn't been acting satisfied with those 5 oz? What to do!?!?!...Right after shes done feeding she wants more..sometimes i give her 1 ounce or 2 if she wants it..and the doc says its ok to give a couple ounces between feedings!?!?..HELP i need advice..should I up her to 6 oz or what?....her borthday is 9-20-07 by the way.