Linea Nigra??

For those who don't know this is the dark line that may or may not show up during pregnancy. It runs along the center of your belly. For those that have one how long is it?? I have a strange very dark dip right under my belly. Its only an inch maybe a little longer. I have no clue what it is but I know its always been there just not dark. Im thinking it could be this line? But if it is why is it so short? I plan on asking my OB but im just wondering if anyone else experienced this.