What Causes A Small Painful Bump In The Belly Area?

I ve experienced this the first in about fifth grade and I m now a junior in high school. It only happens rarely and randomly and it usually would happen rarely when I would hold in pee and didn t get to the bathroom yet. A small bump that I m pretty sure isn t visible comes above my belly button area and there s only a little pain and most of the time when I press on it, it will like fade away but I m just not sure what it is and now that I m older I finally thought to figure out what it is. Any help?