Fatty foods can trigger IBS, maybe look into that or food allergies? Matilda has always had diarrhoea but more so if she has McD's or anything. When we found out about her allergies, and we cut them out, she still has issues when having fatty foods.... we just try to limit her diet and cut out sugar and excessive fat in order to keep her more "regular". I personally would go to the Dr in order to get on top of it.