Doctors are not sure what causes colic, though some speculate it could be the result of a baby's sensitive temperament or immature nervous and digestive systems. These things may make a baby cry easily and have trouble stopping. (Luckily, as babies grow and develop, they are better able to control their crying). However, there are some underlying conditions that can make the crying worse, including:

Gastro esophageal reflux disease (GERD)

GERD often goes hand in hand with colic, and is the first thing a pediatrician will look for. GERD is different than GER, which is simply reflux, a common condition in infants.  GER occurs because the valve-like band of muscle joining the esophagus to the stomach is immature, allowing stomach contents and acids to flow back up into the esophagus.  The result is usually just harmless spit-up, although sometimes baby can be irritable after feedings, cry when placed on his back, and cough or wheeze. GERD, which is estimated to affect five percent of babies, occurs when GER causes more serious complications like inability to gain weight due to persistent rejection of food, projectile vomiting, spit-up that is green or yellow or may even be bloody, difficulty breathing, or trouble with diarrhea.

Although many pediatricians will simply prescribe antacids if GERD is suspected, one way to confirm GERD is to have your doctor do an esophageal pH monitoring test. Basically, she’ll place a tiny tube in your baby’s esophagus and leave it there for 12 to 24 hours, so it can measure the amount of stomach acid that’s regurgitated. While this procedure isn’t necessary to diagnosis GERD, it’s a very reliable way of confirming it. Your baby will likely have to stay overnight in the hospital. If crying coincides with episodes of reflux, then GERD is the likely culprit.

Formula or milk allergy
Colic can also be exacerbated by abdominal pain related to allergies, especially those to infant formula or to traces of dairy products in a breastfeeding mother's milk. Other potential "fuss foods" that are likely part of Mom’s meals include wheat and vegetables, such as broccoli and cauliflower, which can produce painful gas in your babe. An infant who swallows too much air during feeding can wail in discomfort, too. Oftentimes, food allergies and reflux can occur together; signs of an allergy include constipation, diarrhea, and a red circular rash around the anus. 

System Overload
A third possible factor in colic is hyper-excitability and difficulty adjusting to life outside the womb. For some babies, the sensation of the womb needs to continue a bit longer. You will likely develop your own set of comforting techniques, from rocking, singing, swaying and shushing.

Getting Days and Nights Mixed Up
In utero, babies are often lulled to sleep by Mom’s movements during the day, and are more wakeful at night (remember those 3am kicks to the ribs?) Once they’re born, it can take a few weeks to learn that nighttime is for sleeping.

Other Possibilities
Many conditions have symptoms that are easily confused with colic, including ear infections, urinary-tract infections, constipation, and an overly-tight rectal sphincter. These last two may be the problem if a baby grimaces, gets red in the face, or draws his legs up to a distended abdomen before moving his bowels, cries while he poops, and seems greatly relieved afterward.