you should bring your baby to the doctor together with
the diaper. The doctor will have a physical examination on his/her tummy. It happened to my girl several times. It
may be because they have an infection on their guts,
because
1. their hands are dirty when they suct
2. your nipple is not clear enough when you feed them,,
3. the acessory (milk bottle, pump) are not clean enough
.....
But it is ok. They have to remove all the dirt out of their
tummy and it is a physiological process.
But it would be a serious problem of their guts if the poopoo they made is like 'strawberry jam'. You must go to the doctor immediately.