Depo-Shot Questioning

So Ive been on the depo since August 27 this year. And I havnt had a period since. I got my second shot on time. And Im due to get my third in the beginning of Febuary. But yesterday I started spotting for the first time since Ive been on it. And I still am today. At first it was a redish color, and now its a dark brownish color? And my lower stomach area keeps hurting. The pain will come and go, and its sorta like a cramp feeling but different. Should I worry? Or call my doctor? I was just weirded out because I havnt bled or anything till now, and Im having that pain here and there.