birth control pill

I just started the birth control pill Microgestin a month after having my baby. My gyno told me to take it immediately and not to wait until my first period came back. I've been taking it for a week exactly now and have like black spotting. It's really strange. I'm wondering if anyone else has experienced this? Is it normal? If no one can help me, I will just have to call my doc. I wanted to check here first though in case I'm just overreacting.