Because it somewhat resembles a button and it is on your belly.
This is called the pyloric region. A sphincter called the pyloric sphincter is found there.
Abdomen
Umbilical Knot 'Navel' is more common The "Navel" is the area around the "belly button"
Umbilical knot
A belly button ring is a body piercing placed in a cauterized area of the belly button.
Umbilical Hernia. Not particularly dangerous, but get it checked out. You'll experience soreness and tenderness around the belly button area, and it's possible your belly button might swell and protrude a little. Usually results from injury or impact to the area.
belly button
The region near the belly button is called the umbilical region, named after the umbilical cord that connected the fetus to the mother during pregnancy. This area is also known as the navel or the "belly button" and is located in the center of the abdomen.
A belly piercing is a piercing on the belly button. Where your belly button might go in or out, it is the slight arch you get just above it. it is a piercing through that area. depending on how your belly button is formed. Search google for images.
I wish I had a definite answer for you. I'm five weeks today and my belly button is killing me! With my last three pregnancies it was inside the belly button, this one its the area all around the belly button...
If you pick at your belly button that is probably the cause, it is most likely just bruised. If you dont pick at it then you should probably see your doctor as it could be an infection ir hernia.