It happens post delivery with every one. Infact I had so much on my neck. It had turned all black after the delivery and few months before that only I think

Hyperpigmentation is a common, usually harmless condition in which patches of skin become darker in color than the normal surrounding skin. This darkening occurs when an excess of melanin, the brown pigment that produces normal skin color, forms deposits in the skin.. A spot that is a few shades darker than your natural skin color will usually fade within 6 to 12 months. If the color lies deep in your skin, however, fading can take years.