Low birth weight baby

I gave birth to my daughter two weeks ago and she weighed 5 pounds 4 ounces. We weren't expecting her to be so tiny as the ultrasound tech measured her to be about 6 pounds about 2 weeks before she was born. I had her at 39 weeks. She's healthy, just small. The doctor isn't concerned yet but she only gained an ounce at her 2 week appointment. I guess I'm looking for some reassurance that everything will be okay. I'm blaming myself and thinking what could I have done while I was pregnant to make her bigger.. I gained about 35 pounds while pregnant and ate 3 meals a day with snacks of course. Is there anyone that has had a small baby had these same feelings? Did your baby gain weight easily or are they still small? Thanks everyone.