BM vs formula

my baby is 4 months plus now. she's entirely on BM. I only give her formula 1 time in a week, when we bring her out. recently I realize she prefers formula over BM now. she can finished up her formula, when it comes to BM, she will leave behind 1-2 oz. she is drinking 4 Oz now. at times she will just drink 1 Oz of BM and get fussy if we continue to feed her. I also had to watch over my diet because her tummy will get upset if I take in diary products or I ate food that will cause wind to her tummy. am not sure this causes my BM tasteless and make her no appetite. anyone faces the same situation? any advise? thanks