So I Am 17 And I've Been On Depo For

So I am 17 and I've been on depo for about 6 months now. My first three months were smooth, light spotting or no blood at all then my second shot, I got black blood.. I thought it would last a week or so but it got heavier and it's really uncomfortable for me because I don't know what's going on with my body. And it's not more so blood but it's more so black discharge and I am really sore below.. Please help.