Advice for breastfed baby not gaining weight...?

My baby girl is just over 3 weeks and she lost about 10% of her birth weight in the hospital and went to the pediatrician this past Thursday (2 weeks after being discharged) and has not gained ANY weight not even going back to her birth weight. I do not want to give her any bottles or any formula at this point if I can help it so I am looking for any one with any other types of advice like changing my diet or feeding techniques anything. This is my 3rd and I have successfully nursed my other 2 and they never had any weight gaining issues and she seems to eat the same way. She eats every 2-3 hours for a good 20-30 minutes. And she has plenty of wet and poopy diapers same type of stool as my other 2 had the yellow runny mustard looking stool. I am just a bit concerned and would like any advice you have or any one have any similar stories? Thanks! Also, I did ask a ? like this a couple days ago and didn't get too many answers so if you are a rereader sorry.