Could be a lot of things, like others have said. From your VERY brief description I would say kidney related. (stones, infection, nephritis, blah, blah, blah)
First off, don't ask for a diagnosis on a board like this, see a doctor it could be serious...or lead to something serious.
A complete Hx would be needed, such as:
age
medications
any medical conditions
any recent trauma
sexually active
change in urination habit(s), color, odor
pain change with palpation
pain change with movement
discoloration
etc, etc, etc.
It takes a lot to figure out with abd problems, and usually lab tests are required to figure them out or eliminate things it cannot be.
See a doc...