Whether your baby is on exclusive breastfeeding? If yes, how much is his urine output? If he is passing more than 6 times good amount of urine, feeding well, activity good & exclusively breastfed, nothing to bother much. But if he is getting formula, outside milk that too in bottle, he is probably suffering from gastrointestinal infection & needs to be varified by pediatrician.
Next Steps
get stool routine test done & if it shows any pus cells, consult pediatrician.