Anyone know what this might be?

Randi • In love with my best friend. ❤ 5/26/18 👰🤵 Mama to the perfect baby boy born 11/14/19👶💙

It has slowly gotten darker over the past day or so, doesn't seem to bother him, and doesn't have any texture. He's got a doctor's appt in a couple days and I'm not too concerned, just wondering if anyone might know what it is! He's EBF now, but had formula the first couple days of his life if that makes any difference. 😊