Usually digestive problems. The above advice of seeing if you've added something new to your diet is wise. You can try a good blend of digestive enzymes from a good health food store, and aloe juice can be helpful, but removing the culprit from your diet is best. But there are other possibilities as well, including hormonal imbalance, stress, eating too fast, not chewing food enough, lying down too soon after eating, thyroid problems, etc.
It could be any number of things. Usually the most common reason for something like this is a change in diet, if this is not the case, you can go to your doctor and see what he/she thinks. Also, there are over the counter items you can buy for this. A doctor would be the best option. There will be people to answer after me though, so maybe they can answer better than I can.