Hiccups are caused by irritation to the diaphragm, the wide muscle that separates the thoracic (chest) cavity and the abdominal cavity. It is also responsible for pulling the lungs down and out to fill them with air, or to compress them to expel air. Anywhere in the process of respiration, the diaphragm can get irritated to the point of muscular excitation- it will twitch, causing the hiccup. Chances are it's a passing thing, but if it persists for any thing longer than a day something wierd is happening. Alcohol or spicy food can also cause it to to ape-poop, or an upper esophogeal obstruction, but that would likely be accompanied by pain and mild nausea. Prolonged hiccupping can result in sleep and feeding disturbances, GERD, aspiration of stomach contents, and esophageal varices (varicose veins in the food tube).
I gotta hear that again...
Try slowly breathing in to capacity, letting out just a bit of air, and bearing down like you're trying to crap. At 5 second intervals, let some air out untill you're empty or you feel the need to breathe. If any pain or dizziness occur while doing this, STOP IMMEDIATELY. If you start to see little sparkly floaties, STOP IMMEDIATELY. If the problem persists more than 12-24 hours, seek medical attention.