Why is only one nipple dark??? TMI pic

*boobie pic don't open if it will upset you*

For awhile now I've noticed one of my nipples, the nipple not the areola, is darker than the other???? Is this normal??

It turns back to a pinker color when I lay down or if they get perky or something. But when they are soft and I'm walking around is a dark color.

I switched between three different birth controls in a small time frame and that's when I started to notice it.

Should I be worried?