What Does Black Blood Bleeding With Severe Abdominal Cramps While On Shots Mean?

I am on the shot and have been for a year. I have not had a period in about 6 months. Last week I was having severe abdominal cramps and started bleeding. I consulted my town's family planning clinic where I recieve my shots, they told me this happens sometimes with the shots. But I am bleeding black blood and it is very thick, it looks like tar. Should I worry or is this normal?