Have you ever poked your finger in there?  Really crammed it down?  Feels weird, right?  There's a reason for that.

All placental mammals have navels.  It's the point of contact for the umbilical cord which handles all nutrient and waste issues for a developing fetus.  The navel is essentially a scar.

When you poke your finger in there really deep, you stimulate some nerves that your brain interprets as being somewhere around your bladder.