Do you have any other symptoms? Nausea? Cough?
Does anything make the pain better or worse? Putting pressure on the area? Hot or cold compress? Eating a fatty meal?
There are a lot of things that can cause abdominal pain this severe. The main things that come to mind are gallstones, kidney stones, appendicitis, and pleurisy (I'm sure there are other things, too).
I would make an emergency appointment with your regular doctor (or just walk in if you have to) and have him or her examine you. If your doctor thinks you need to go back to the hospital, you will get better care if you were admitted by your doctor rather than just went into the ER on your own (sad, but true).
When you go in, make sure you can tell your doctor what makes the pain better or worse (besides the pain killers), what brought it on (if anything), what you did right before it happened, if you have been having any other symptoms, etc.