Many causes. The answer depends on whether you have other symptoms along with the vomiting. There are many causes for this including: infection, stomach flu, food poisoning, ulcers, reflux disease, hiatal hernia, gastroparesis, head injury, hepatitis, gallbladder, pancreatitis, heart attack, side effects of medications...As always, if it's happening all the time and it's not normal, it needs to be checked out.
Reflux. Gerd or gastroesophageal reflux disease is a common cause of belching. Occasionally you can feel "heartburn" or throat irritation because of this. Avoiding certain foods and eating smaller portions can help reduce symptoms. In some cases medications such as priolsec or Zantac (ranitidine) are recommended.