Why would urine output drop?

Gravity? ...Ha ha, no seriously - I had a kidney stone and my urine output decreased. Soon (like 12 hours into it) I felt the pressure in my back and side, even into my gut. The pain is due to the pressure of urine in the kidneys with no place to go, because the kidney stone blocks the little tube from the kidney to the bladder. When that little sucker won't budge on its own under the pressure it's serious. This is just one possible explanation for decreased urine output of course, any other ideas anyone?