Switching formula

Hi FTM here, my 4 weeks LO is having Lactogen 1 currently, ytd was the first time I try to offer him Nan Optipro twice out of 8 feedings. And I realise that he is having diarrhoea, stool is mustard yellowish and smelly. Does it mean that the Formula does not suit him? LO did not vomit or reject when feeding, pls advice if I should continue giving the new formula ? I’m so worry when I see him having diarrhoea :( #firsttimemom