It sounds to me like a kidney stone. It could also be a UTI. First things first, drink lots of water. Also, go see a doctor to rule out a urinary tract infection.

I have a history of kidney stones... mine never show up on xrays or ultrasounds or CT scans, so just because a scan is clear doesn't mean there isn't a little stone there.

Lots of water and dilluted cranberry juice.... Generally, if there is blood but not an infection, it is a stone.