4month Old Baby Refusing To Drink Milk.

Hi so my baby have been refusing to drink milk after mid week of her 3rd month and she just turned 4th 2days ago. I tried every ways to feed her and finally found 1 she liked. But now even after putting her in her favourite position she don't wanna drink anymore and this is quite frustrating and worrying as she can drink 300-400ml a day. Last time she used to drink more but now it's getting lesser and lesser.. Any advice or help? She's been sleeping alot too...