What Causes Intermenstrual Bleeding While On Implanon?

I'm 17 years old. I had the implanon implant fitted 2 years ago June. For the last 6 - 8 months I've been bleeding between periods but its really dark blood and clots. I've been tested for chlamydia and that was negative, I've also been having abdomin and pelvic pains. Not sure whats going on with me, could you help ?