I Have The Implant Birth Control In My Arm So

I have the implant birth control in my arm so I don't really have a period much.. but I had one on March and then one last month it last like 3 days was gone for 2 and came back for 2 weeks I have stopped bleeding for about 2 weeks. well a couple days ago I starting have a really dark brown discharge some time come in chunks like a blood clot but they are so dark it almost looks black. and now my stomach is hurting and I have burning when I go to the bathroom and it hurts to wipe I notice that it swollen. what could this be?