What causes the urge to defecate?

I've been feeling the urge the defecate constantly, sometimes 6-8 hours after previous defecation. Going to the toilet doesn't work, as nothing comes out. My regular defecation schedule is 24-36 hours. Only then can feces come out.

Does anybody have this as well? A constant feeling of a need to defecate, even though you don't really have to go? What causes this? What's the science behind it?