Breastfeeding and weight gain

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My son is 2 weeks old and is exclusively breastfed (I pump). According to how many diapers he goes through, how much he eats, etc, he should be gaining weight. But he’s not. Anyone possibly have any insight about that?? He was 8lbs 12oz at birth, had to spend a few days in the nicu (had fluid in his lungs) and when we took him home, he weighed around 7lbs. Now he weighs 8lbs 2oz.