Hi, I'm 29y Old Mom Of 3y Old Son. We

Hi, I'm 29y old mom of 3y old son. We live in USA. I was very fair before my son was born. After delivery my face becomes darker and dull. Even I got pigmentation around my mouth area. I'm a stay at home mom, so I don't think it's a cause of sun damage. My period is normal, I don't have any help problem. So what is the reason and how do I get my fair complaxion back.