baby drinks less milk

hi mommies, mahina mgdede ang 4 month old baby girl ko... most of the time less than 20 oz lng naiinom nya in a day. active na man sya and pasok pa na man sa normal ang weight nya kaya lng borderline na sa pagiging underweight. any tips? ps formula-fed sya kc nawala ang breasmilk ko when i got hospitalized due to UTI, I'm currently on my relactation journey ?