please help me.. baby not eating?

I am a first time mom to a beautiful baby girl born almost three weeks ago. For the past few days she has not been drinking more than 1-2 ounces each feeding when she was drinking 3 since she was a week old. She just falls asleep eating. We just changed her formula last Wednesday thinking that would change (it helped with her spit up which was very bad) and today we switched her bottle to Dr. Browns from Avent and she is still doing the same. I feel like I am doing something wrong and my baby is not getting enough food. What can I do?