Hard to poop

Good evening! I really need an advice about my baby, she's 16 months and recently we're having problems on her poop. Nahihirapan syang mag poop, never kami nagpalit ng milk nya, she has this behavior na pag alam nyang need nyang umire, pinipigilan nya hanggang sa maging iretable sya tsaka sya magpu poop pag sobrang hilab na ng tummy nya. Need your advice or tips.para po hindi na maulit. thanks!