What Causes High Temperature Along With Diarrhoea In An Infant?

My 10 month old has diarrhea and a temperature that started off at 100 this morning and is currently 101.2. He has had infant tylenol (2.5ml dose) at 8:30 am and 1pm (east coast time). I was wondering what could be causing the temperature and when should I start to seek medical help?