Why would a breastfed baby suddenly lose weight?

she may not have enough milk for the baby and may need to supplement with formula and just use the formula when needed and at 6 months she should start eating more solids at age 6months the babys iron supply is depleted from birth so she will need iron through her foods iron fortified formula or cereal. My sister was breastfeeding her baby when he was born he lost weight and kept losing it and they had to use baby formula and breastmilk bc my sis wasnt making enough milk so that could be why.