Welcome to HCM,
You are pregnant and during this period pigmentation can come in various parts of body as you must have noticed in other areas also,this is called cloasma, many times neck and face known as Mask of pregnancy
You should avoid exposure to sunlight and use sunscreen lotion with good SPF.
usually these pigmented area dis appear in 3 months after delivery.
In rare case they persist also hence please do not worry but please consult your Dermatologist
just to make sure.
You should not feel embarrassed about anything please do ask your doctor also He/She will be happy to help you.
Best of luck