Why would a baby pot belly pig have diarrhea?

This could be due to a number of reasons, either the piglet is sick, the mother's feed has been changed and is affecting the piglet through the breast milk, if it is just weaned off of the mother and is being fed a new feed it may be causing some irritation in the digestive system, or there are environmental changes such stress, weather, or other factors. It is best to contact your veterinarian to give it medication and to conclude what factor is causing the diarrhea.