Unpredictable diarrhoea

Hi I'm 15 and have had a problem for about 2 years now, where I get really random attacks of diarrhoea. They only last for a few hours, but during that time I effectively glued to the toilet. It only happens ones or twice a month, however it is very embarrassing and inconvenent when it does happen, especially in school. Their isn't any prior warning, just a slight tummy ache followed by a very sudden urge to poo. Their have been times when I have run out of class to make it to the toilet I actually pooed myself for the first time probably since I wore nappies, last week!! After that my mum insisted I saw a doctor. All he has said is to not eat any junk food for the ext few weeks and if it is still a problem then to come back.
Anyone got anything similar or any advice?