Possibly extreme constipation.  I had what I thought was appendicitis a few weeks ago.  A constant, non-fluctuating, un-moving pain in my lower right abdomen.  It lasted for about 5 hours and then I was able to get to sleep.  It didn't hurt in the morning, but my muscles were sore from the cramping it caused for about a week after.  I never figured out exactly what it was, but I think it might have been constipation, heh.