My 40 days old daughter is on bf and fm both as I had low milk supply but from last one week she is spitting breastmilk and Is ok with fm what could be the reason?

has your diet changed which could impact your bm? is she just spitting out small amounts or puking the whole thing out?

A baby can get used to a particular taste and since formula is sweeter and much more tastier than breast milk she is showing her preference. Work towards increasing your supply by feeding often or pumping. If you are feeding formula with a bottle then discontinue that as well since the suckling and swallowing style at breast and bottle is also different and can cause confusion for the baby.