Hi, personally I had not experience this but I believe it's not un-usual.
As our hormones changes during pregnancy, that's where some women notice that they have a fine hairline running from the centre of their chests down to the belly button and as the pregnancy goes on, that line gets darker and more obvious. And some of us actually has bad/better skin complexion/hair or/and very obvious stretch marks.
I believe in your case, the dark skin on your bum cheeks will go away a few months after delivery of your baby.
In the meantime, relax and be happy