That's wayyy too general. A bump on the stomach could be a rash, a pimple, a tumor or an insect bite. To know for sure (and if its causing you problems), please contact a health...
this is called "crap i got a rash"
Some people wear underwear that has padding in the butt. But others were just born that way.
That is your body's way of telling you to go see a doctor.
If you have that, go to the doctor and get it checked.