Linea nigra and late period.

Lily-Anne
Do these have any relation to eachother? 
My period is three weeks late, and I've had this line under my belly button for about three weeks now as well. I wouldn't even glance twice at it if I had had one before, but I haven't and I don't understand why it isn't going away? 
I know it's to do with an increase in Melanin, but what could have caused that? 
It's not very dark, but it's been getting darker as I'm rather fair skinned, 
I have posted about this before, but I really am confused as to why it hasn't gone away yet. Is there anyone that has the same problem?