Could be IBS, or it could just be the way you are, and not a sign of illness, if it isn't diarrhea.
It might even be healthier. There is a reflex that makes one go after eating, and yours might be stronger than most. It usually gets less when one grows up, but not always.
Sometimes you can get a bacteria in your intestines that will cause this, so you need to visit a doctor who can do a stool sample and draw blood, to see if you've got something they can give you medicine for to kill the bacteria. Also, the doc can check and see if you've got some sort of physical problem that is causing this need to go after you eat. But if they cannot find anything, they can give you a prescription to soothe the bowels. And if you are on any medications for whatever, you should have the doc review the side effects, in case those are doing this, and he can switch kinds for you. He can also maybe refer you to an allergist, to see if something you eat all the time is doing this. In the meantime, if you have a diarrhea type waste, then there are over-the-counter solutions to that, until you can get to a doc.