Skin discolouration

Hello ladies!! I am having an issue and need some help. So my husband has been telling me that the back of my neck is getting dark. Me thinking he's joking haven't taken it seriously until he took a picture of it and showed me!!! It look horrible to me!!! So my question is, is this normal? What can I do to clear it up? Why is it darkening? I'm about 6-7 weeks from my blood work.