Breastfeeding question!

My baby is now 4 weeks old, was born 5 weeks premature I’ve been strictly breastfeeding. She’s eating sometimes every hour to two hours. I haven’t been pumping because she’s on the breast so much. When we went for our 2 week check up she had only gained an oz. We were suppose to go back the 18th but had to cancel it due to scheduling issues. I’m so afraid she hasn’t gained much weight because she’s still so tiny! Do I have any other moms that have tiny breastfed babies? Ones that didn’t weigh very much weight in the first month or so?