Hi Jamnik,
Not sure if it's still called Linea Nigra if it's on your head, but I have the one on my belly (and mine's crooked!!! Either that or my belly button is off centre!!!), and I have really patchy, dark pigmentation on my face now. Apparently it's fairly common and hormone related. Maybe this is what you're experiencing?
Not sure if that helps at all
All the best,
Lily