There are many organs in the abdomen. Pain in the abdomen can originate from any one of them, including:

Organs related to digestion -- the stomach, the end of the esophagus, the small and large intestines, the liver, the gallbladder, and the pancreas.
      The aorta -- a large blood vessel that runs straight down the inside of the abdomen.
      The appendix -- an organ in the lower right abdomen that no longer serves much function. 
      The kidneys -- two bean shaped organs that lie deep within the abdominal cavity.

However, the pain may originate from somewhere else -- like your chest or pelvic region. You may also have a generalized infection affecting many parts of your body, like the flu or strep throat.

The intensity of the pain does not always reflect the seriousness of the condition causing the pain. Severe abdominal pain can be from mild conditions, such as gas or the cramping of viral gastroenteritis. On the other hand, relatively mild pain or no pain may be present with life-threatening conditions, such as cancer of the colon or early appendicitis.

Causes

Many different conditions can cause abdominal pain. The key is to know when you must seek medical care right away. In many cases you can simply wait, use home care remedies, and call your doctor at a later time only if the symptoms persist.

1. Possible causes include:
2. Excessive gas
   Chronic constipation
   Lactose intolerance (milk intolerance)
   Viral gastroenteritis (stomach flu)
   Irritable bowel syndrome
   Heartburn or indigestion
   Gastroesophageal reflux
   Ulcers
   Cholecystitis (inflammation of the gallbladder) with or without gallstones
   Appendicitis (inflammation of the appendix)
   Diverticular disease, including inflammation of small pouches that form in the large intestines (diverticulitis)
   Bowel obstruction -- in addition to pain, this causes nausea, bloating, vomiting, and inability to pass gas or stool
   Food allergy
   Food poisoning (salmonella, shigella)
   Hernia
   Kidney stones
   Urinary tract infections
   Pancreatitis (inflammation of the pancreas)
   Intussusception (telescoping intestines) -- while uncommon, this is a serious possible cause of pain in an infant who may be drawing his or knees to the chest and crying to indicate the pain
   Dissecting abdominal aortic aneurysm -- bleeding into the wall of the aorta
   Parasite infections (Giardia)
   Sickle cell crisis
   Crohn's disease or ulcerative colitis (two different types of inflammatory bowel disease)
   Epiploic appendagitis

When an inflamed organ in the abdomen ruptures or leaks fluid, you not only have excruciating pain, your abdomen will be very stiff (board-like) and you will likely have a fever. This occurs when you have peritonitis due to an infection spreading in the abdominal cavity from the ruptured organ, like the appendix. This is a medical emergency.

In infants, prolonged unexplained crying (often called "colic") may be caused by abdominal pain that may end with the passage of gas or stool. Colic is often worse in the evening. Cuddling and rocking the child may bring some relief.

Abdominal pain that occurs during menstruation may be from menstrual cramps or it may indicate a problem in a reproductive organ. This includes conditions such as endometriosis (when tissue from the uterus is displaced to somewhere else like the pelvic wall or ovaries), uterine fibroids (thick bands of muscular and fibrous tissue in the uterus), ovarian cysts, ovarian cancer (rare), or pelvic inflammatory disease (PID) -- infection of the reproductive organs, usually from a sexually transmitted disease.

Abdominal pain may actually be caused by an organ in the chest, like the lungs (for example, pneumonia) or the heart (like a heart attack). Or, it may stem from a muscle strain in the abdominal muscles.