What Causes Darkening Of Lips And Neck During Pregnancy?

Hi, I am 35 weeks pregnant and I have notice over the couse of two weeks my lips and neck on becoming darker. My lip have always been pink, but now they appear black. Likewise with my neck, it has always been the same color of my skin, but now there's black rings appearing. This have never happen during my previous three pregnancies...what could cause the color of my lips and neck to turn black?