I've been doing rolls on harder surfaces (such as non-carpeted floors and concrete) from time to time, and the thing that causes me to go ouch most often, is... the belt I wear around my pants/jeans/whatever it is at the moment !
The pain comes from it pressing against my kidney(s) at the point in time when that point of my back makes contact with the surface...
I'm not sure I understand. The kidneys are above the pelvic bone, and most people wear their pants in such a manner where their belts are at or below their pelvis. When wearing a dogi, however, the obi is usually positioned right on the kidneys. You say "around my pants/jeans/whatever" so are you wearing your dogi while practicing ukemi on hard surfaces or are you wearing your pants too high?