How old is the baby? It could be anything from poorly sterilised bottles, to stomach bugs, to lactose intolerance. Maybe this article would help... https://sg.theasianparent.com/diarrhoea-in-children/
Diarrhoea in babies needs to be checked asap. Is your baby on solids? I'd so, it could be food poisoning. However, there's a higher chance it's a tummy bug that needs proper treatment.
It can go from food poisoning to indigestion. really depends on how severe it is.
could be some infection. if it gets worse, see your pedia.