Hi Please Help,my Normally Healthy 2years6months Old Son Developed A

Hi please help,my normally healthy 2years6months old son developed a severe chest infection a month ago and was treated successfully with amoxicillin.however 10 days ago he started having diarrhoea with u digested food and is off his food,considering he normally has an excellent appetite.he is constantly crying and I noticed his abdomen is dissented.