Dark patch on neck (front)

Hi, I'm having dark patches on front of my neck. It suddenly appeared overnight. I had not noticed it before. Im currently 23 weeks and my next gynae appt is 2 weeks away. Just curious if other mummies had this and does it mean anything?? Is it a sign of gestational diabetes, quite worried? Note: I didn't apply anything on neck except sunscreen. #pleasehelp #firstbaby #advicepls #firstmom #firsttimemom