If this has been going on for years, this is most likely not a concern. In some patients, this is called enhanced gastric-intestine reflex, which means your colon is activated once you get something into the stomach. If you take a look at the stool and find that it is the same with a normal stool with regard to stool color and characteristics (solid vs watery), then it is not a concern. Additionally, there should be no blood in the stool. If there is blood in the stool, it is definitely abnormal.
If it just occurs out of the blue, it may mean something. Diarrhea soon after eating can be caused by many diseases, such as food poisoning.