Can my breastmilk not be enough?

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My 4 month d is mix fed. She gets mostly breast milk with 2 bottle of formula at nigbt ( around 180-200mls). Shes been growing on the 25th centile since birth. Shes at the breast quite frequently during the day. I am worried that my breastmilk isnt nutriitios enough as shes gaining weight so slowly about 100g a week.has anyone experienced this? Should I increase ber formula feeds?