baby sembilit

hi dears baby sy baru 4 bulan.. masa fully bf baby sy tak de constipation skr dah mix FM kesian dia kena sembilit.. 1St I try s26 tapi muka dia naik merah2..lepas itu I try novalac isinova sama juga sembilit.. nw I use novalac shj.. nak tukar susu.. susu ape bagus.. plz suggest me