Abdominal (belly) pain after eating is a common problem. While there are some very serious causes of this, there are some also very minor causes. There are many things we don't know about this, so unfortunately, it is difficult to answer your question. Given your Crohn's I would strongly recommend that you discuss this with your gastroenterologist
(stomach specialist) as he or she may have some insight in your specific situation.
The digestive process is a complex one. In order to digest and absorb food, our body relies on an intricate balance of mechanical motion, open pipes, the correct acid/base balance and the correct enzymes. As such, there are many things that can go wrong and cause pain. For example, if there is excess acid in the stomach, this can spill to the esophagus and cause heartburn. How these processes remain in balance is something we do not understand fully.
It is common for different people to have different foods they cannot tolerate. While some of this is understood ( gluten products for people with Celiac disease, milk products for those with lactose intolerance) much of it isn't. That is why we simply recommend avoiding foods your body doesn't "agree with."
If you are having problems with a specific food that is affecting your overall nutrition, talk to your gastroenterologist. While difficult to pinpoint why it is, he or she may have some insight depending on the location and nature of your Crohn's.